MkDocs.yml: replace deprecated pages with nav

Author: mloskot

mkdocs.yml

@@ -3,7 +3,7 @@ site_description: SOCI - The C++ Database Access Library
 repo_url: https://github.com/SOCI/soci/
 copyright: Copyright &copy; 2017 <a href="https://github.com/msobczak">Maciej Sobczak</a> and <a href="http://soci.sourceforge.net/people.html">SOCI Team</a>.
 
-pages:
+nav:
   - Home: index.md
   - Overview:
     - Getting Started: quickstart.md