fix http1 metadata (#6086)

Co-authored-by: Stavros Kontopoulos <[email protected]>

Author: knative-prow-robot

docs/serving/services/http-protocol.md

@@ -76,7 +76,7 @@ metadata:
   namespace: default
 spec:
   template:
-    spec:
+    metadata:
       annotations:
         features.knative.dev/http-full-duplex: "Enabled"
 ...