chore(AIP-214): bold requirement word "must" (#1408)

Author: zchenyu

aip/general/0214.md

@@ -25,7 +25,7 @@ library.
 
 1.  APIs wishing to convey an expiration **must** rely on a
     [`google.protobuf.Timestamp`][timestamp] field called `expire_time`.
-2.  APIs wishing to allow a relative expiration time must define a `oneof`
+2.  APIs wishing to allow a relative expiration time **must** define a `oneof`
     called `expiration` (or `{something}_expiration`) containing both the
     `expire_time` field and a separate [`google.protobuf.Duration`][duration]
     field called `ttl`, the latter marked as input only.