📝(backend) clarify trailing slash requirement in API Swagger doc

Specify that POST routes require a trailing slash to avoid
confusion and incorrect usage from API consumers.

docs/openapi.yaml

@@ -190,6 +190,7 @@ paths:
         '403':
           $ref: '#/components/responses/ForbiddenError'
 
+  /rooms/:
     post:
       tags:
         - Rooms

docs/resource_server.yaml

@@ -113,6 +113,7 @@ paths:
         '403':
           $ref: '#/components/responses/ForbiddenError'
 
+  /rooms/:
     post:
       tags:
         - Rooms