io: fix spelling in documentation for io.Discard
In the process of refactoring ioutil.Discard to io.Discard in
CL 263141 "an" should have been changed to "a" but was likely
missed in the process.
This commit corrects the spelling of the documentation.
Change-Id: I0609c45878291f8f01560efc3f3e6fba191e095b
GitHub-Last-Rev:
e3257ca272dff42ed7d07b6e6a5fc49493772653
GitHub-Pull-Request: golang/go#45190
Reviewed-on: https://go-review.googlesource.com/c/go/+/304209
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>