Ensure that /usr/local/etc/pkg/repos exists

The directory /usr/local/etc/pkg/repos doesn't exist by default, so have
it created in an example that will  otherwise fail.

PR:		253351

documentation/content/en/books/handbook/ports/_index.adoc

@@ -302,6 +302,7 @@ To switch from quarterly to latest run the following commands:
 
 [source,bash]
 ....
+# mkdir -p /usr/local/etc/pkg/repos
 # cp /etc/pkg/FreeBSD.conf /usr/local/etc/pkg/repos/FreeBSD.conf
 ....