Update docs/source/contributing/contributing.md

Co-Authored-By: KirstieJane <kw401@cam.ac.uk>

docs/source/contributing/contributing.md

@@ -74,7 +74,7 @@ These are not hard rules to be enforced by 🚓 but they are suggestions written
 * **Keep your PR focused.**
   The best PRs solve one problem.
   If you end up changing multiple things, please open separate PRs for the different conceptual changes.
-* **Add tests your code.**
+* **Add tests to your code.**
   PRs will not be merged if Travis is failing.
 * **Apply [PEP8](https://www.python.org/dev/peps/pep-0008/)** as much as possible, but not too much.
   If in doubt, ask.