doc: fix http and http2 writeEarlyHints() parameter

Both http and http2 response.writeEarlyHints() take an object, not an array, as their first parameter. For http, this was updated in the examples via #44820 except for the final example, which this patch fixes. The doc for the http2 version was not touched in #44820 although I am pretty sure from skimming the code that it behaves identically to http, and so propose to change its doc as well. Finally, some bogus headline levels are fixed in http2 docs.