Remove &mut from io::read_to_string signature

`@m-ou-se` [realized][1] that because `Read` is implemented for `&mut impl
Read`, there's no need to take `&mut` in `io::read_to_string`.

Removing the `&mut` from the signature allows users to remove the `&mut`
from their calls (and thus pass an owned reader) if they don't use the
reader later.

[1]: rust-lang#80218 (comment)

Author: camelid

library/std/src/io/mod.rs

@@ -1031,14 +1031,14 @@ pub trait Read {
 ///
 /// # use std::io;
 /// fn main() -> io::Result<()> {
-///     let stdin = io::read_to_string(&mut io::stdin())?;
+///     let stdin = io::read_to_string(io::stdin())?;
 ///     println!("Stdin was:");
 ///     println!("{}", stdin);
 ///     Ok(())
 /// }
 /// ```
 #[unstable(feature = "io_read_to_string", issue = "80218")]
-pub fn read_to_string<R: Read>(reader: &mut R) -> Result<String> {
+pub fn read_to_string<R: Read>(mut reader: R) -> Result<String> {
     let mut buf = String::new();
     reader.read_to_string(&mut buf)?;
     Ok(buf)