layouts: update partials to _partials

Hugo v0.146.0 introduced a
[new template system](https://gohugo.io/templates/new-templatesystem-overview/)
that changed the structure of the `layouts` directory.

Commit https://github.com/jpanther/congo/commit/99d5cf8b28ce82377588310799420721943a3e65
updated this theme to support the new `layouts` directory structure but
missed a few updates to `templates.Exists` calls. This patch fixes those
calls.

layouts/baseof.html

@@ -21,7 +21,7 @@
       >
     </div>
     {{ $header := print "header/" .Site.Params.header.layout ".html" }}
-    {{ if templates.Exists ( printf "partials/%s" $header) }}
+    {{ if templates.Exists ( printf "_partials/%s" $header) }}
       {{ partial $header . }}
     {{ else }}
       {{ partial "header/basic.html" . }}