io: tweak Reader comment

I think this has the same meaning as before,
but the text is tighter, and it makes some people happy.

Fixes #10182.

Change-Id: I7ee1eae4bcd6ee4a5898ea948648939e6bde5f01
Reviewed-on: https://go-review.googlesource.com/11674
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/io/io.go

@@ -54,7 +54,7 @@ var ErrNoProgress = errors.New("multiple Read calls return no data or error")
 // An instance of this general case is that a Reader returning
 // a non-zero number of bytes at the end of the input stream may
 // return either err == EOF or err == nil.  The next Read should
-// return 0, EOF regardless.
+// return 0, EOF.
 //
 // Callers should always process the n > 0 bytes returned before
 // considering the error err.  Doing so correctly handles I/O errors