all: fix misuses of "a" vs "an"

all: fix misuses of "a" vs "an"

Fixes the misuse of "a" vs "an", according to English grammatical
expectations and using https://www.a-or-an.com/

Change-Id: Ic9600dcbb3d843880349729478266c4b9bcf7316
Reviewed-on: https://go-review.googlesource.com/c/go/+/531335
Run-TryBot: shuang cui <imcusg@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>