path/filepath: fixed misaligned comment.

The comment for 'Clean' function is prepended with spaces instead of
a single tab, resulting in visually misaligned comment in the generated
documentation.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/73840043

src/pkg/path/filepath/path.go

@@ -67,7 +67,7 @@ const (
 //	   along with the non-.. element that precedes it.
 //	4. Eliminate .. elements that begin a rooted path:
 //	   that is, replace "/.." by "/" at the beginning of a path,
-//         assuming Separator is '/'.
+//	   assuming Separator is '/'.
 //
 // The returned path ends in a slash only if it represents a root directory,
 // such as "/" on Unix or `C:\` on Windows.