doc: avoid colloquialism

I witnessed an English-as-a-second-language speaker puzzle over the word
"borked" in our documentation today. Reword to avoid colloquialisms.
It's more precise and clear this way anyway. Also reformat `Optional:`
to be consistent with how it appears elsewhere in the doc.

Author: Trott

COLLABORATOR_GUIDE.md

@@ -345,7 +345,7 @@ Additionally:
 
 ### Technical HOWTO
 
-_Optional:_ ensure that you are not in a borked `am`/`rebase` state
+Clear any `am`/`rebase` that may already be underway.
 
 ```text
 $ git am --abort