net/http: fix a typo in comments

HTTP is an initialism, not an acronym, where you pronounce each letter as a
word. It's "an H", not "a H".

Running `find src/net/http -type f | xargs grep -n 'an HTTP' | wc -l` shows
that the "an HTTP" form is used 67 times across the `net/http` package.
Furthermore, `find src/net/http -type f | xargs grep -n 'a HTTP' | wc -l`
yields only 4 results.

Change-Id: I219c292a9e2c9bf7a009dbfe82ea8b15874685e9
GitHub-Last-Rev: 6ebd095023af47444b6b0fc5b6d7b26d85f4c7b7
GitHub-Pull-Request: golang/go#33810
Reviewed-on: https://go-review.googlesource.com/c/go/+/191700


Reviewed-by: Toshihiro Shiino <shiino.toshihiro@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>