Fix async-function doc broken link in swagger

The documentation for async-function pointed to
the old faas/guide and should be to docs.openfaas.com

Signed-off-by: gabrielsson <max.gabrielsson@gmail.com>

api-docs/swagger.yml

@@ -160,7 +160,7 @@ paths:
     post:
       summary: 'Invoke a function asynchronously in OpenFaaS'
       description: >-
-        See https://github.com/openfaas/faas/blob/master/guide/asynchronous.md.
+        See https://docs.openfaas.com/reference/async/.
       parameters:
       - in: path
         name: functionName