chore(AIP-100): replace Hangouts with Chat (#1414)

Author: kenjitoyama

aip/general/0100.md

@@ -268,7 +268,7 @@ A few tips:
 
 ### ...if one of my API reviewers is unresponsive?
 
-Reach out to the reviewer on Hangouts. If that fails, reach out to the other
+Reach out to the reviewer on Chat. If that fails, reach out to the other
 reviewer, who will coordinate accordingly. If that fails also, escalate
 according to [AIP-1][].