Clean up E0712 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0712.md

@@ -1,5 +1,5 @@
-This error occurs because a borrow of a thread-local variable was made inside a
-function which outlived the lifetime of the function.
+A borrow of a thread-local variable was made inside a function which outlived
+the lifetime of the function.
 
 Erroneous code example: