Step 3 in renaming step-22->step-31 and the other way around

git-svn-id: https://svn.dealii.org/trunk@15800 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/examples/step-xx/Makefile b/deal.II/examples/step-31/Makefile
similarity index 100%
rename from deal.II/examples/step-xx/Makefile
rename to deal.II/examples/step-31/Makefile

diff --git a/deal.II/examples/step-xx/doc/intro.dox b/deal.II/examples/step-31/doc/intro.dox
similarity index 100%
rename from deal.II/examples/step-xx/doc/intro.dox
rename to deal.II/examples/step-31/doc/intro.dox

diff --git a/deal.II/examples/step-xx/doc/results.dox b/deal.II/examples/step-31/doc/results.dox
similarity index 100%
rename from deal.II/examples/step-xx/doc/results.dox
rename to deal.II/examples/step-31/doc/results.dox

diff --git a/deal.II/examples/step-xx/step-22.cc b/deal.II/examples/step-31/step-22.cc
similarity index 100%
rename from deal.II/examples/step-xx/step-22.cc
rename to deal.II/examples/step-31/step-22.cc