chore(AIP-162): fix markdown nested list (#1450)

Author: luangong

aip/general/0162.md

@@ -66,8 +66,8 @@ message BookRevision {
 - The `message` name **must** be named `{ResourceType}Revision`.
 - The resource revision **must** contain a field with a message type of the
   parent resource, with a field name of `snapshot`.
-    - The value of `snapshot` **must** be the configuration of the parent
-      at the point in time the revision was created.
+  - The value of `snapshot` **must** be the configuration of the parent
+    at the point in time the revision was created.
 - The resource revision **must** contain a `create_time` field (see [AIP-142][]).
 - The resource revision **may** contain a repeated field `alternate_ids`, which would
   contain a list of resource IDs that the revision is also known by (e.g. `latest`)