Fix pre-commit hook so it doesn't catch .latex+lhs extensions.

CONTRIBUTING.md

@@ -225,7 +225,7 @@ A good way to ensure no new warnings are introduced is to use a Git
 before creating a commit:
 
     #!/bin/sh
-    git diff --cached --name-only | grep '.hs$' | xargs hlint
+    git diff --cached --name-only | grep '\.hs$' | xargs hlint
 
 Saving this to `.git/hooks/pre-commit`, and making the script
 executable, will prevent accidental introduction of potentially