Documentation: AsciiDoc spells em-dash as double-dashes, not triple

Again, we do not usually process release notes with AsciiDoc, but it
is better to be consistent.

This incidentally reveals breakages left by an ancient 5e00439f
(Documentation: build html for all files in technical and howto,
2012-10-23).  The index-format documentation was originally written
to be read as straight text without formatting and when the commit
forced everything in Documentation/ to go through AsciiDoc, it did
not do any adjustment--hence the double-dashes will be seen in the
resulting text that is rendered as preformatted fixed-width without
converted into em-dashes.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-fetch.txt b/Documentation/git-fetch.txt
index 8deb614..ee51c1a 100644
--- a/Documentation/git-fetch.txt
+++ b/Documentation/git-fetch.txt
@@ -71,7 +71,7 @@
 * When `git fetch` is run without specifying what branches
   and/or tags to fetch on the command line, e.g. `git fetch origin`
   or `git fetch`, `remote.<repository>.fetch` values are used as
-  the refspecs---they specify which refs to fetch and which local refs
+  the refspecs--they specify which refs to fetch and which local refs
   to update.  The example above will fetch
   all branches that exist in the `origin` (i.e. any ref that matches
   the left-hand side of the value, `refs/heads/*`) and update the