Keep consistency in example for Stdin StdinLock

Stdin uses handle whereas StdinLock uses stdin_lock, changed it to handle.

library/std/src/io/stdio.rs

@@ -251,8 +251,8 @@ pub struct Stdin {
 ///     let mut buffer = String::new();
 ///     let stdin = io::stdin(); // We get `Stdin` here.
 ///     {
-///         let mut stdin_lock = stdin.lock(); // We get `StdinLock` here.
-///         stdin_lock.read_to_string(&mut buffer)?;
+///         let mut handle = stdin.lock(); // We get `StdinLock` here.
+///         handle.read_to_string(&mut buffer)?;
 ///     } // `StdinLock` is dropped here.
 ///     Ok(())
 /// }