Add missing quotes for the types in comments.

Author: tomasz-rozanski

src/std/str.md

@@ -119,7 +119,7 @@ Or maybe you want an array of bytes that's mostly text? Byte strings to the resc
 use std::str;
 
 fn main() {
-    // Note that this is not actually a &str
+    // Note that this is not actually a `&str`
     let bytestring: &[u8; 20] = b"this is a bytestring";
 
     // Byte arrays don't have the `Display` trait, so printing them is a bit limited
@@ -136,7 +136,7 @@ fn main() {
     let raw_bytestring = br"\u{211D} is not escaped here";
     println!("{:?}", raw_bytestring);
 
-    // Converting a byte array to str can fail
+    // Converting a byte array to `str` can fail
     if let Ok(my_str) = str::from_utf8(raw_bytestring) {
         println!("And the same as text: '{}'", my_str);
     }
@@ -147,7 +147,7 @@ fn main() {
     // Bytestrings don't have to be UTF-8
     let shift_jis = b"\x82\xe6\x82\xa8\x82\xb1\x82"; // "ようこそ" in SHIFT-JIS
 
-    // But then they can't always be converted to str
+    // But then they can't always be converted to `str`
     match str::from_utf8(shift_jis) {
         Ok(my_str) => println!("Conversion successful: '{}'", my_str),
         Err(e) => println!("Conversion failed: {:?}", e),