Fix style and grammar

Author: chrisvittal

src/librustc_typeck/diagnostics.rs

@@ -4627,7 +4627,7 @@ crate's documentation.
 
 E0642: r##"
 This error indicates that there is a mismatch between generic parameters and
-impl Trait parameters in a trait Declaration versus it's impl.
+impl Trait parameters in a trait declaration versus its impl.
 
 ```compile_fail,E0642
 #![feature(conservative_impl_trait)]