grammar typo caught by axkr

Author: rocky

mathics/builtin/functional/apply_fns_to_lists.py

@@ -180,7 +180,7 @@ class MapAt(Builtin):
     >> MapAt[f, {a, b, c}, 2]
      = {a, f[b], c}
 
-    Above, we specified a simple integer value 2. In general, the expression can an arbitrary vector.
+    Above, we specified a simple integer value 2. In general, the expression can be an arbitrary vector.
 
     Using 'MapAt' with 'Function[0]', we can zero a value or values in a vector: