chore(AIP-9): fix API interface heading level (#1546)

Author: noahdietz

aip/general/0009.md

@@ -47,7 +47,7 @@ API frontend server is often called an API proxy.
 away from each other. In some cases, they can be compiled into a single
 application binary and run inside a single process.
 
-#### API interface
+### API interface
 
 The element of an API specification IDL that groups API methods, such as a
 Protocol Buffers `service` definition. It is typically mapped to a similar high