Fix typo in CONTRIBUTING.md

Thanks to @nemequ

Closes compiler-explorer#1815

Author: AbrilRBS

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Contributing to Compiler Explorer
 
-First of, if you're reading this: thank you! Even considering contributing to
+First off, if you're reading this: thank you! Even considering contributing to
  **Compiler Explorer** is very much appreciated!
 Before we go too far, an apology: **Compiler Explorer** grew out of a bit of
  hacky JavaScript into a pretty large and well-used project pretty quickly.