fix typo

Author: poolio

python/thunder/imgprocessing/registration.py

@@ -6,7 +6,7 @@ class Registration(object):
     """
     Class for constructing registration algorithms.
 
-    Construct a registration algorthm by specifying its name,
+    Construct a registration algorithm by specifying its name,
     and passing any additional keyword arguments. The algorithm
     can then be used to fit registration parameters on an Images object.