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.

src/io/io.go

@@ -566,7 +566,7 @@ func (t *teeReader) Read(p []byte) (n int, err error) {
 	return
 }
 
-// Discard is an Writer on which all Write calls succeed
+// Discard is a Writer on which all Write calls succeed
 // without doing anything.
 var Discard Writer = discard{}