Documentation/SubmittingPatches: unify whitespace/tabs for the DCO

The Developers Certificate of Origin has a mixture of tabs and white
spaces which is annoying to view if your editor explicitly views white
space characters.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -254,15 +254,15 @@ pretty simple: if you can certify the below:
             person who certified (a), (b) or (c) and I have not modified
             it.
 
-	(d) I understand and agree that this project and the contribution
-	    are public and that a record of the contribution (including all
-	    personal information I submit with it, including my sign-off) is
-	    maintained indefinitely and may be redistributed consistent with
-	    this project or the open source license(s) involved.
+        (d) I understand and agree that this project and the contribution
+            are public and that a record of the contribution (including all
+            personal information I submit with it, including my sign-off) is
+            maintained indefinitely and may be redistributed consistent with
+            this project or the open source license(s) involved.
 
 then you just add a line saying
 
-	Signed-off-by: Random J Developer <random@developer.example.org>
+        Signed-off-by: Random J Developer <random@developer.example.org>
 
 This line can be automatically added by Git if you run the git-commit
 command with the -s option.