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/7503Reviewed-by: Rob Pike <r@golang.org>

src/flag/flag.go

@@ -441,9 +441,9 @@ 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"
+			s += "\n    \t"
 		}
 		s += usage
 		if !isZeroValue(flag.DefValue) {

src/flag/flag_test.go

@@ -380,20 +380,20 @@ func TestHelp(t *testing.T) {
 
 const defaultOutput = `  -A	for bootstrapping, allow 'any' type
   -Alongflagname
-   	disable bounds checking
+    	disable bounds checking
   -C	a boolean defaulting to true (default true)
   -D path
-   	set relative path for local imports
+    	set relative path for local imports
   -F number
-   	a non-zero number (default 2.7)
+    	a non-zero number (default 2.7)
   -G float
-   	a float that defaults to zero
+    	a float that defaults to zero
   -N int
-   	a non-zero int (default 27)
+    	a non-zero int (default 27)
   -Z int
-   	an int that defaults to zero
+    	an int that defaults to zero
   -maxT timeout
-   	set timeout for dial
+    	set timeout for dial
 `
 
 func TestPrintDefaults(t *testing.T) {