bytes: correct tense in comment

Undo incorrect change accidentally made in CL 299109.

Change-Id: Iba29827d0fbd3637c311cebc50c2f4ea437fc582
Reviewed-on: https://go-review.googlesource.com/c/go/+/301830
Trust: Ian Lance Taylor <[email protected]>
Reviewed-by: Tobias Klauser <[email protected]>

Author: ianlancetaylor

src/bytes/buffer.go

@@ -387,7 +387,7 @@ var errUnreadByte = errors.New("bytes.Buffer: UnreadByte: previous operation was
 
 // UnreadByte unreads the last byte returned by the most recent successful
 // read operation that read at least one byte. If a write has happened since
-// the last read, if the last read returned an error, or if the read reads zero
+// the last read, if the last read returned an error, or if the read read zero
 // bytes, UnreadByte returns an error.
 func (b *Buffer) UnreadByte() error {
 	if b.lastRead == opInvalid {