Clean up E0637 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0637.md

@@ -1,6 +1,7 @@
 An underscore `_` character has been used as the identifier for a lifetime.
 
-Erroneous example:
+Erroneous code example:
+
 ```compile_fail,E0106,E0637
 fn longest<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
          //^^ `'_` is a reserved lifetime name
@@ -11,13 +12,15 @@ fn longest<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
     }
 }
 ```
+
 `'_`, cannot be used as a lifetime identifier because it is a reserved for the
 anonymous lifetime. To fix this, use a lowercase letter such as 'a, or a series
 of lowercase letters such as `'foo`.  For more information, see [the
 book][bk-no].  For more information on using the anonymous lifetime in rust
 nightly, see [the nightly book][bk-al].
 
 Corrected example:
+
 ```
 fn longest<'a>(str1: &'a str, str2: &'a str) -> &'a str {
     if str1.len() > str2.len() {