doc: omit needless "for"

What was intended was perhaps "... plumbing does for you" ("you" added), but
simply omitting the word "for" is more terse and gets the intended point across
just as well, if not more so.

I originally went with the approach of writing "for you", but Junio C
Hamano suggested this approach instead.

Signed-off-by: Kristoffer Haugsbakk <kristoffer.haugsbakk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitcore-tutorial.txt

@@ -25,7 +25,7 @@ you want to understand Git's internals.
 The core Git is often called "plumbing", with the prettier user
 interfaces on top of it called "porcelain". You may not want to use the
 plumbing directly very often, but it can be good to know what the
-plumbing does for when the porcelain isn't flushing.
+plumbing does when the porcelain isn't flushing.
 
 Back when this document was originally written, many porcelain
 commands were shell scripts. For simplicity, it still uses them as