git add -i: replace \t with blanks in the help message

Within the help message of 'git add -i', the 'diff' command uses one
tab character and blanks to create the space between the name and the
description while the others use blanks only.  So if the tab size is
not at 4 characters, this description will not be in range.
Replace the tab character with blanks.

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-add--interactive.perl

@@ -1566,7 +1566,7 @@ status        - show paths with changes
 update        - add working tree state to the staged set of changes
 revert        - revert staged set of changes back to the HEAD version
 patch         - pick hunks and update selectively
-diff	      - view diff between HEAD and index
+diff          - view diff between HEAD and index
 add untracked - add contents of untracked files to the staged set of changes
 EOF
 }