time: fix incorrect "zero padding" comments

The comment on invalid time values in Constants and example
refers to _ zero padding when it should refer to space padding.

Change-Id: I5784356e389d324703e20eec6203f147db92880f
Reviewed-on: https://go-review.googlesource.com/75410Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/example_test.go

@@ -318,7 +318,7 @@ func ExampleParse() {
 	fmt.Println(t)
 
 	// Some valid layouts are invalid time values, due to format specifiers
-	// such as _ for zero padding and Z for zone information.
+	// such as _ for space padding and Z for zone information.
 	// For example the RFC3339 layout 2006-01-02T15:04:05Z07:00
 	// contains both Z and a time zone offset in order to handle both valid options:
 	// 2006-01-02T15:04:05Z

src/time/format.go

@@ -19,7 +19,7 @@ import "errors"
 // the same transformation to a general time value.
 //
 // Some valid layouts are invalid time values for time.Parse, due to formats
-// such as _ for zero padding and Z for zone information.
+// such as _ for space padding and Z for zone information.
 //
 // Within the format string, an underscore _ represents a space that may be
 // replaced by a digit if the following number (a day) has two digits; for