Clean up E0502 explanation

GuillaumeGomez · GuillaumeGomez · commit 9db736059372 · 2020-04-03T13:41:34.000+02:00
diff --git a/src/librustc_error_codes/error_codes/E0502.md b/src/librustc_error_codes/error_codes/E0502.md
@@ -1,5 +1,4 @@
-This error indicates that you are trying to borrow a variable as mutable when it
-has already been borrowed as immutable.
+A variable already borrowed as immutable was borrowed as mutable.
 
 Erroneous code example:
 
