flag: use four spaces before tab, not three

Four spaces is what works well for both 4- and 8-space tab stops.

Screen with fixed-width font and 4-space tab stops:
http://imgur.com/lps5Lbb

Change-Id: I7d2b813d674c3e0a68f79d63bc5d5ec5bd4f87bb
Reviewed-on: https://go-review.googlesource.com/7503
Reviewed-by: Rob Pike <r@golang.org>

src/flag/flag.go

@@ -441,7 +441,7 @@ func (f *FlagSet) PrintDefaults() {
 		if len(s) <= 4 { // space, space, '-', 'x'.
 			s += "\t"
 		} else {
-			// Three spaces before the tab triggers good alignment
+			// Four spaces before the tab triggers good alignment
 			// for both 4- and 8-space tab stops.
 			s += "\n    \t"
 		}