doc: fix code nits in common/README

1. Sync comments and code
2. Fix typos

Backport-PR-URL: #20456
PR-URL: #17971
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/common/README.md

@@ -610,7 +610,7 @@ frame.
 // padlen is an 8-bit integer giving the number of padding bytes to include
 // final is a boolean indicating whether the End-of-stream flag should be set,
 // defaults to false.
-const data = new http2.DataFrame(id, payload, padlen, final);
+const frame = new http2.DataFrame(id, payload, padlen, final);
 
 socket.write(frame.data);
 ```
@@ -631,8 +631,7 @@ The `http2.HeadersFrame` is a subclass of `http2.Frame` that serializes a
 // padlen is an 8-bit integer giving the number of padding bytes to include
 // final is a boolean indicating whether the End-of-stream flag should be set,
 // defaults to false.
-const data = new http2.HeadersFrame(id, http2.kFakeRequestHeaders,
-                                    padlen, final);
+const frame = new http2.HeadersFrame(id, payload, padlen, final);
 
 socket.write(frame.data);
 ```