[SRVKS-1260] Correct field name in HTTP/1 sample

Author: ochromy

knative-serving/http-configuration/http1-full-duplex-support.adoc

@@ -23,7 +23,7 @@ metadata:
   namespace: default
 spec:
   template:
-    spec:
+    metadata:
       annotations:
         features.knative.dev/http-full-duplex: "Enabled"
 ...