Tidied up the long error description

src/librustc_error_codes/error_codes/E0637.md

@@ -1,6 +1,5 @@
-
 An underscore `_` character or a numeric literal `u8`, `i32`, `f64`, etc has
-been used as the identifier for a lifetime. 
+been used as the identifier for a lifetime.
 
 Erroneous code example 1:
 ```
@@ -29,6 +28,4 @@ fn some_function<'a>(str1: &'a str, str2: &'a str) -> &'a str {
     //Some code
 }
 ```
-
 [bk-no]: https://doc.rust-lang.org/book/appendix-02-operators.html#non-operator-symbols
-