Clean up E0518 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0518.md

@@ -1,7 +1,7 @@
-This error indicates that an `#[inline(..)]` attribute was incorrectly placed
-on something other than a function or method.
+An `#[inline(..)]` attribute was incorrectly placed on something other than a
+function or method.
 
-Examples of erroneous code:
+Example of erroneous code:
 
 ```compile_fail,E0518
 #[inline(always)]