Update base64.md not to use deprecated functions

[jungseoklee]

This patch addresses the following warnings:

  warning: use of deprecated function `base64::encode`: Use Engine::encode
   --> src/bin/base64.rs:3:14
    |
  3 | use base64::{encode, decode};
    |              ^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

  warning: use of deprecated function `base64::decode`: Use Engine::decode
   --> src/bin/base64.rs:3:22
    |
  3 | use base64::{encode, decode};
    |                      ^^^^^^

  warning: use of deprecated function `base64::encode`: Use Engine::encode
   --> src/bin/base64.rs:7:17
    |
  7 |   let encoded = encode(hello);
    |                 ^^^^^^

  warning: use of deprecated function `base64::decode`: Use Engine::decode
   --> src/bin/base64.rs:8:17
    |
  8 |   let decoded = decode(&encoded)?;
    |                 ^^^^^^

Also, this patch updates broken links on decode and encode.

🎉 Hi and welcome! Please read the text below and remove it - Thank you! 🎉

No worries if anything in these lists is unclear. Just submit the PR and ask away! 👍

---

Things to check before submitting a PR

• the tests are passing locally with cargo xtask test all
• commits are squashed into one and rebased to latest master
• PR contains correct "fixes #ISSUE_ID" clause to autoclose the issue on PR merge
  • if issue does not exist consider creating it or remove the clause
• non rendered items are in sorted order (links, reference, identifiers, Cargo.toml)
• links to docs.rs have wildcard version https://docs.rs/tar/*/tar/struct.Entry.html
• example has standard error handling
• code identifiers in description are in hyperlinked backticks

[`Entry::unpack`]: https://docs.rs/tar/*/tar/struct.Entry.html#method.unpack

Things to do after submitting PR

• check if CI is happy with your PR

Thank you for reading, you may now delete this text! Thank you! 😄