Fixes gpg.format default value

The `gitformat-signature` docs page refers to a `gpg` value that
does not work as expected. Using the value `opengpg` instead, as
documented under https://git-scm.com/docs/git-config `gpg.format`
section fixes it.

Author: ruyadorno

external/docs/content/docs/gitformat-signature.html

@@ -46,7 +46,7 @@ <h2 id="_description"><a class="anchor" href="#_description"></a>DESCRIPTION</h2
 </div>
 <div class="dlist">
 <dl>
-<dt class="hdlist1" id="Documentation/gitformat-signature.txt-codegpgcodePGP"> <a class="anchor" href="#Documentation/gitformat-signature.txt-codegpgcodePGP"></a><code>gpg</code> (PGP) </dt>
+<dt class="hdlist1" id="Documentation/gitformat-signature.txt-codegpgcodePGP"> <a class="anchor" href="#Documentation/gitformat-signature.txt-codegpgcodePGP"></a><code>opengpg</code> (PGP) </dt>
 <dd>
 <p><code>-----BEGIN PGP SIGNATURE-----</code> and <code>-----END PGP SIGNATURE-----</code>.
 Or, if gpg is told to produce RFC1991 signatures,