docs: small reformulation

doc/source/index.rst

@@ -6,7 +6,7 @@ Kwant documentation
    :end-before: See also the files
 
 Researchers, please note that even though :doc:`Kwant is Free Software
-<license>`, scientific integrity requires to give appropriate credit.  If you
+<license>`, scientific integrity obliges to give appropriate credit.  If you
 write a scientific paper whose results have been obtained with the help of
 Kwant, please :doc:`acknowledge <acknowledgements>` the work of the :doc:`people
 that have developed it <authors>`.