multiple typo fixes

CITING

INSTALL

doc/source/images/graphene.py.diff

doc/source/images/plot_graphene.py.diff

doc/source/pre/whatsnew/1.0.rst

doc/source/reference/kwant.graph.rst

doc/source/tutorial/graphene.py

doc/source/tutorial/introduction.rst

doc/source/tutorial/plot_graphene.py

doc/source/tutorial/plot_zincblende.py

doc/source/tutorial/tutorial1.rst

doc/source/tutorial/tutorial2.rst

doc/source/tutorial/tutorial3.rst

doc/source/tutorial/tutorial4.rst

doc/source/tutorial/tutorial5.rst

doc/source/tutorial/tutorial6.rst