Clean up E0517 explanation

GuillaumeGomez · GuillaumeGomez · commit eed4b219e243 · 2020-04-14T13:26:17.000+02:00
diff --git a/src/librustc_error_codes/error_codes/E0517.md b/src/librustc_error_codes/error_codes/E0517.md
@@ -1,5 +1,4 @@
-This error indicates that a `#[repr(..)]` attribute was placed on an
-unsupported item.
+A `#[repr(..)]` attribute was placed on an unsupported item.
 
 Examples of erroneous code:
 
