docs/admin_api: fix header level on 'Users' page

Signed-off-by: Sumner Evans <sumner@beeper.com>

docs/admin_api/user_admin_api.md

@@ -1180,7 +1180,7 @@ The following parameters should be set in the URL:
 - `user_id` - The fully qualified MXID: for example, `@user:server.com`. The user must
   be local.
 
-### Check username availability
+## Check username availability
 
 Checks to see if a username is available, and valid, for the server. See [the client-server 
 API](https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-register-available)
@@ -1198,7 +1198,7 @@ GET /_synapse/admin/v1/username_available?username=$localpart
 The request and response format is the same as the
 [/_matrix/client/r0/register/available](https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-register-available) API.
 
-### Find a user based on their ID in an auth provider
+## Find a user based on their ID in an auth provider
 
 The API is:
 
@@ -1237,7 +1237,7 @@ Returns a `404` HTTP status code if no user was found, with a response body like
 _Added in Synapse 1.68.0._
 
 
-### Find a user based on their Third Party ID (ThreePID or 3PID)
+## Find a user based on their Third Party ID (ThreePID or 3PID)
 
 The API is: