Remove tiny typo

Excuse the small PR, I just saw the typo while browsing through the documentation.

beem/snapshot.py

@@ -249,7 +249,7 @@ class AccountSnapshot(list):
 
             :param array only_ops: Limit generator by these
                 operations (*optional*)
-            :param array exclude_ops: Exclude thse operations from
+            :param array exclude_ops: Exclude these operations from
                 generator (*optional*)
 
         """