Fix numbered list in AIP-4222 (#1223)

Author: vchudnov-g

aip/client-libraries/4222.md

@@ -51,6 +51,7 @@ rpc CreateTopic(CreateTopicRequest) {
 ```
 
 The value of the field `field` **must** be one of the following:
+
 1. a name of a field in the top-level of the request message
 1. a dot-separated path of field names leading to a field in a sub-message of
 the request message e.g. `"book.author.name"` where `book` is a message field in