Fix logo at navbar

by run

~~~
$ find _includes -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
$ find _layouts -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
$ find _extras -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
$ find _episodes -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
$ find bin -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
$ find assets -type f -exec sed -i "s/page.carpentry/site.carpentry/g" {} \;
~~~