path/filepath: fixed misaligned comment.
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://codereview.appspot.com/73840043

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh