typo fix (#410)

Author: Delcior

PyMPDATA/impl/grid.py

@@ -8,7 +8,7 @@
 
 
 def make_chunk(span, n_threads, jit_flags):
-    """returns an njit-ted function which returns the subdomain extend for a
+    """returns an njit-ted function which returns the subdomain extent for a
     given thread, for static grid size no calculations are done at run-time"""
     static = span > 0