correct typo

content/extensions.rst

@@ -23,7 +23,7 @@ Generating quasicrystals
 
 Code for reproducing numerics from the paper "`Aperiodic Weak Topological Superconductors <https://arxiv.org/abs/1510.06035>`_"
 by Fulga et al.
-Contains functionality for building 2D Ammann-Beenaker tilings, an example of a quasicrystal, and
+Contains functionality for building 2D Ammann-Beenker tilings, an example of a quasicrystal, and
 Kwant systems constructed from such tilings.
 
 Time-dependent transport