Update PR template to try and avoid /kind being automatically set

We recently updated the PR template to include some useful examples of
the /kind command. Unfortunately Prow will see those even though they're
inside a comment and apply them to the PR.

This commit removes the valid kind examples and replaces them with some
more text hopefully explaining enough how to use them.

Author: Scott

.github/pull_request_template.md

@@ -6,10 +6,11 @@
 Describe your changes here- ideally you can get that description straight from
 your descriptive commit message(s)! 
 
-In addition, categorize the changes you're making using the "/kind" Prow command, such as:
-/kind bug
-/kind documentation
-/kind feature
+In addition, categorize the changes you're making using the "/kind" Prow command, example:
+
+/kind <kind>
+
+Supported kinds are: bug, cleanup, design, documentation, failing-test, feature, flake, misc, question, tep
 -->
 
 # Submitter Checklist