Fix URL in docu for opensearch.
diff --git a/doc/config.rst b/doc/config.rst
index cc35a75..a11254e 100644
--- a/doc/config.rst
+++ b/doc/config.rst
@@ -707,7 +707,7 @@
 
 .. confval:: html_use_opensearch
 
-   If nonempty, an `OpenSearch <http://opensearch.org>` description file will be
+   If nonempty, an `OpenSearch <http://opensearch.org>`_ description file will be
    output, and all pages will contain a ``<link>`` tag referring to it.  Since
    OpenSearch doesn't support relative URLs for its search page location, the
    value of this option must be the base URL from which these documents are