Fix Http2Headers.method(...) javadocs (#9718)

Motivation:

The javadocs of Http2Headers.method(...) are incorrect, we should fix these.

Modifications:

Correct javadocs

Result:

Fixes https://github.com/netty/netty/issues/8068.

codec-http2/src/main/java/io/netty/handler/codec/http2/Http2Headers.java

@@ -136,7 +136,7 @@ public interface Http2Headers extends Headers<CharSequence, CharSequence, Http2H
     Iterator<CharSequence> valueIterator(CharSequence name);
 
     /**
-     * Sets the {@link PseudoHeaderName#METHOD} header or {@code null} if there is no such header
+     * Sets the {@link PseudoHeaderName#METHOD} header
      */
     Http2Headers method(CharSequence value);