[examples] Remove reference to deleted attachHeaders from HeaderClientInterceptor

[sergiitk]

grpc-java/examples/src/main/java/io/grpc/examples/header/HeaderClientInterceptor.java

Lines 52 to 57 in b170334

	public void onHeaders(Metadata headers) {
	/**
	* if you don't need receive header from server,
	* you can use {@link io.grpc.stub.MetadataUtils#attachHeaders}
	* directly to send header
	*/

io.grpc.stub.MetadataUtils#attachHeaders has been removed in #10443.