github: Update PR template for new "Patches" content

The old "Patches" wiki page is not available anymore. Now that the
content has been integrated with the submitting_patches document,
reference that instead.

Signed-off-by: Tom Rini <[email protected]>
Reviewed-by: Simon Glass <[email protected]>

Author: trini

.github/pull_request_template.md

@@ -1,6 +1,6 @@
 Please do not submit a Pull Request via github.  Our project makes use of
 mailing lists for patch submission and review.  For more details please
-see https://www.denx.de/wiki/U-Boot/Patches
+see https://u-boot.readthedocs.io/en/latest/develop/sending_patches.html
 
 The only exception to this is in order to trigger a CI loop on Azure prior
 to posting of patches.