Fix doc link used in suggestion for pinning self

[Trivaxy]

Fixes #147901

The old suggestion linked to a documentation page that no longer exists. Presumably, this is the better place to link to now

[rustbot]

r? @SparrowLii

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[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

diff of stderr:

5    |         ^^^^ method not found in `impl Future<Output = ()>`
6    |
7    = help: method `poll` found on `Pin<&mut impl Future<Output = ()>>`, see documentation for `std::pin::Pin`
-    = help: self type must be pinned to call `Future::poll`, see https://rust-lang.github.io/async-book/04_pinning/01_chapter.html#pinning-in-practice
+    = help: self type must be pinned to call `Future::poll`, see https://rust-lang.github.io/async-book/part-reference/pinning.html
9 help: consider pinning the expression
10    |
11 LL ~     let mut pinned = std::pin::pin!(fut);


The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args async-await/issue-108572.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/async-await/issue-108572.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/async-await/issue-108572" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2021"
stdout: none
--- stderr -------------------------------
error[E0599]: no method named `poll` found for opaque type `impl Future<Output = ()>` in the current scope
##[error]  --> /checkout/tests/ui/async-await/issue-108572.rs:12:9
   |
LL |     fut.poll(cx);
   |         ^^^^ method not found in `impl Future<Output = ()>`
   |
   = help: method `poll` found on `Pin<&mut impl Future<Output = ()>>`, see documentation for `std::pin::Pin`
   = help: self type must be pinned to call `Future::poll`, see https://rust-lang.github.io/async-book/part-reference/pinning.html
help: consider pinning the expression
   |
LL ~     let mut pinned = std::pin::pin!(fut);
LL ~     pinned.as_mut().poll(cx);
   |

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0599`.