all: fix article typos

a -> an

Change-Id: I7d0413396e51f16e0a83732a07a5536548e03506
Reviewed-on: https://go-review.googlesource.com/63992
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: namusyaka

idna/idna.go

@@ -69,7 +69,7 @@ var lockTestDurations = []time.Duration{
 // lockTestNames are the names of a set of mutually compatible locks. For each
 // name fragment:
 //	- _ means no explicit lock.
-//	- i means a infinite-depth lock,
+//	- i means an infinite-depth lock,
 //	- z means a zero-depth lock,
 var lockTestNames = []string{
 	"/_/_/_/_/z",