Correct fast-import timezone documentation.

Andy Parkins and Linus Torvalds both noticed that the description
of the timezone was incorrect.  Its not expressed in minutes.
Its more like "hhmm", where "hh" is the number of hours and "mm"
is the number of minutes shifted from GMT/UTC.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Documentation/git-fast-import.txt

@@ -226,8 +226,8 @@ The time of the change is specified by `<time>` as the number of
 seconds since the UNIX epoc (midnight, Jan 1, 1970, UTC) and is
 written in base-10 notation using US-ASCII digits.  The committer's
 timezone is specified by `<tz>` as a positive or negative offset
-from UTC, in minutes.  For example EST would be expressed in `<tz>`
-by ``-0500''.
+from UTC.  For example EST (which is typically 5 hours behind GMT)
+would be expressed in `<tz>` by ``-0500'' while GMT is ``+0000''.
 
 `from`
 ^^^^^^