Add filetype check to pre hook example

We have `Language: Cpp` in .clang-format but recently it tried to format
my makefile anyways.
Let's add this to be sure.

CONTRIBUTING.md

@@ -15,7 +15,9 @@ You can add the following snippet to `.git/hooks/pre-commit`:
 ```shell
 for f in $(git diff --cached --name-only)
 do
-    clang-format -i $f
+    if [[ "$f" =~ \.(c|h)$ ]]; then
+        clang-format -i $f
+    fi
 done
 ```