minor grammar fixes

src/codegen.md

@@ -39,7 +39,7 @@ There are a few benefits to using LLVM:
   and maintenance burden.
 - We benefit from the large suite of advanced optimizations that the LLVM
   project has been collecting.
-- We automatically can compile Rust to any of the platforms for which LLVM has
+- We can automatically compile Rust to any of the platforms for which LLVM has
   support. For example, as soon as LLVM added support for wasm, voila! rustc,
   clang, and a bunch of other languages were able to compile to wasm! (Well,
   there was some extra stuff to be done, but we were 90% there anyway).

src/conventions.md

@@ -20,7 +20,7 @@ in isolation with `./x.py test src/tools/tidy`.
 
 ### Copyright notice
 
-In the past, files begin with a copyright and license notice. Please **omit**
+In the past, files began with a copyright and license notice. Please **omit**
 this notice for new files licensed under the standard terms (dual
 MIT/Apache-2.0).
 

src/diagnostics.md

@@ -34,7 +34,7 @@ warnings, errors, fatal errors, suggestions, etc.
 [parsesses]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/sess/struct.ParseSess.html
 [session]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/struct.Session.html
 
-In general, there are two class of such methods: ones that emit an error
+In general, there are two classes of such methods: ones that emit an error
 directly and ones that allow finer control over what to emit. For example,
 [`span_err`][spanerr] emits the given error message at the given `Span`, but
 [`struct_span_err`][strspanerr] instead returns a