clean up E0641 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0641.md

@@ -1,19 +1,19 @@
 Attempted to cast to/from a pointer with an unknown kind.
 
-Erroneous code examples:
+Erroneous code example:
 
 ```compile_fail,E0641
 let b = 0 as *const _; // error
 ```
 
-Must give information for type of pointer that is being cast from/to if the
-type cannot be inferred.
+Type information must be provided if a pointer type being cast from/into another
+type which cannot be inferred:
 
 ```
 // Creating a pointer from reference: type can be inferred
-let a = &(String::from("Hello world!")) as *const _; // Ok
+let a = &(String::from("Hello world!")) as *const _; // ok!
 
-let b = 0 as *const i32; // Ok
+let b = 0 as *const i32; // ok!
 
-let c: *const i32 = 0 as *const _; // Ok
+let c: *const i32 = 0 as *const _; // ok!
 ```