Update base64.md not to use deprecated functions

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.

Author: jungseoklee

src/encoding/string/base64.md

@@ -8,12 +8,12 @@ and decodes it with [`decode`].
 ```rust,edition2018
 use anyhow::Result;
 use std::str;
-use base64::{encode, decode};
+use base64::prelude::*;
 
 fn main() -> Result<()> {
     let hello = b"hello rustaceans";
-    let encoded = encode(hello);
-    let decoded = decode(&encoded)?;
+    let encoded = BASE64_STANDARD.encode(hello);
+    let decoded = BASE64_STANDARD.decode(&encoded)?;
 
     println!("origin: {}", str::from_utf8(hello)?);
     println!("base64 encoded: {}", encoded);
@@ -23,5 +23,5 @@ fn main() -> Result<()> {
 }
 ```
 
-[`decode`]: https://docs.rs/base64/*/base64/fn.decode.html
-[`encode`]: https://docs.rs/base64/*/base64/fn.encode.html
+[`decode`]: https://docs.rs/base64/latest/base64/engine/trait.Engine.html#method.decode
+[`encode`]: https://docs.rs/base64/latest/base64/engine/trait.Engine.html#method.encode