all: fix a number of misuses of the word "an"

all: fix a number of misuses of the word "an"

After golang.org/cl/210124, I wondered if the same error had gone
unnoticed elsewhere. I quickly spotted another dozen mistakes after
reading through the output of:

	git grep '\<[Aa]n [bcdfgjklmnpqrtvwyz][a-z]'

Many results are false positives for acronyms like "an mtime", since
it's pronounced "an em-time". However, the total amount of output isn't
that large given how simple the grep pattern is.

Change-Id: Iaa2ca69e42f4587a9e3137d6c5ed758887906ca6
Reviewed-on: https://go-review.googlesource.com/c/go/+/210678
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Zach Jones <zachj1@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>