Change pull_request_template to include spotless formatter instructions

.github/pull_request_template.md

@@ -3,7 +3,7 @@ Please ensure that your pull request meets the following requirements - thanks!
 - Does not contain merge commits. Rebase instead.
 - Contains commits with descriptive titles.
 - New code is written in Kotlin whenever possible.
-- Follows our existing codestyle (`gradlew ktlintCheck`; will be checked by CI).
+- Follows our existing codestyle (`gradlew spotlessCheck` to check and `gradlew spotlessFormat` to format your source code; will be checked by CI).
 - Does not break any unit tests (`gradlew testDebugUnitTest`; will be checked by CI).
 - Uses a descriptive title; don't put issue numbers in there.
 - Contains a reference to the issue that it fixes (e.g. _Closes #XXX_ or _Fixes #XXX_) in the body text.