message: Fix the navindex error in the xml file.
diff: <tab type="mainpage" visible="yes" title=""/>
  <tab type="user" visible="yes" title="Tutorial" url="https://github.com/ginkgo-project/ginkgo/wiki/Tutorial:-Building-a-2D-Poisson-Solver" />
  <tab type="user" visible="yes" title="Examples" url="@ref Examples" />
  - <tab>
  - <tab type="user" visible="yes" title="Using Ginkgo" url="@ref install_ginkgo" />
  + <tab type="user" visible="yes" title="Using Ginkgo" url="@ref install_ginkgo">
  <tab type="user" visible="yes" title="Installing Ginkgo" url="@ref install_ginkgo" />
  <tab type="user" visible="yes" title="Testing Ginkgo" url="@ref testing_ginkgo" />
  <tab type="user" visible="yes" title="Benchmarking Ginkgo" url="@ref benchmarking_ginkgo" />
  