Documentation: remove backslash before ~ in fast-import manual

Use the {tilde} entity to get a literal tilde without fuss.
With \~, asciidoc 8.5.2 (and probably earlier versions) keeps the
backslash in the output.

Reported-by: Frédéric Brière <fbriere@fbriere.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-export.txt

@@ -93,7 +93,7 @@ marks the same across runs.
 [git-rev-list-args...]::
        A list of arguments, acceptable to 'git rev-parse' and
        'git rev-list', that specifies the specific objects and references
-       to export.  For example, `master\~10..master` causes the
+       to export.  For example, `master{tilde}10..master` causes the
        current master reference to be exported along with all objects
        added since its 10th ancestor commit.