sync tutorial names with doc

doc/texinfo/gmsh.texi

tutorial/c++/t1.cpp

tutorial/c++/t10.cpp

tutorial/c++/t13.cpp

tutorial/c++/t2.cpp

tutorial/c++/t7.cpp

tutorial/c++/t8.cpp

tutorial/c++/t9.cpp

tutorial/c/t1.c

tutorial/python/t1.py

tutorial/python/t10.py

tutorial/python/t13.py

tutorial/python/t2.py

tutorial/python/t7.py

tutorial/python/t8.py

tutorial/python/t9.py

tutorial/t1.geo

tutorial/t10.geo

tutorial/t13.geo

tutorial/t2.geo