fix typo

content/doc/index.txt

@@ -2,7 +2,7 @@ Kwant documentation
 ===================
 
 The two available resources for learning Kwant are its documentation, and the
-paper inctroducing Kwant structure and design. The documentation contains
+paper introducing Kwant structure and design. The documentation contains
 tutorial and package reference. The tutorial part of the documentation is
 hands-on, and mainly explains how to use Kwant and its various features. The
 package reference provides detailed description of every Kwant