Documentation/technical/api-config.txt: fix formatting

Asciidoctor versions v1.5.7 or later print the following warning while
building the documentation:

      ASCIIDOC technical/api-config.html
  asciidoctor: WARNING: api-config.txt: line 232: unterminated listing block

This highlight an issue (even with older Asciidoctor versions) where
the length of the '----' lines surrounding a code example don't match,
and the rest of the document is rendered in monospace.

Fix this by making sure that the length of those lines match.

The page rendered with AsciiDoc doesn't have this formatting issue.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-config.txt

@@ -229,7 +229,7 @@ A `config_set` can be used to construct an in-memory cache for
 config-like files that the caller specifies (i.e., files like `.gitmodules`,
 `~/.gitconfig` etc.). For example,
 
----------------------------------------
+----------------------------------------
 struct config_set gm_config;
 git_configset_init(&gm_config);
 int b;