clean up E0404 explanation

src/librustc_error_codes/error_codes/E0404.md

@@ -1,5 +1,5 @@
-You tried to use something which is not a trait in a trait position, such as
-a bound or `impl`.
+A type that is not a trait was used in a trait position, such as a bound
+or `impl`.
 
 Erroneous code example:
 
@@ -18,8 +18,8 @@ struct Foo;
 fn bar<T: Foo>(t: T) {} // error: `Foo` is not a trait
 ```
 
-Please verify that you didn't misspell the trait's name or otherwise use the
-wrong identifier. Example:
+Please verify that the trait's name was not misspelled or that the right
+identifier was used. Example:
 
 ```
 trait Foo {
@@ -32,7 +32,7 @@ impl Foo for Bar { // ok!
 }
 ```
 
-or
+or:
 
 ```
 trait Foo {