doc: fix length of underlined section-title

In AsciiDoc, it is OK to say:

   this is my title
   -------------------------

but AsciiDoctor is more strict. Let's match the underline to
the title (which also makes the source prettier to read).
The output from AsciiDoc is the same either way.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

Documentation/git-fast-import.txt

@@ -600,7 +600,7 @@ be removed from the branch.
 See `filemodify` above for a detailed description of `<path>`.
 
 `filecopy`
-^^^^^^^^^^^^
+^^^^^^^^^^
 Recursively copies an existing file or subdirectory to a different
 location within the branch.  The existing file or directory must
 exist.  If the destination exists it will be completely replaced