fix docs typo

docs/src/undo.md

@@ -1,7 +1,7 @@
 # Undo
 
 It's easy to make a mistake: mark the wrong task as done, or hit enter before noticing a typo in a tag name.
-The `ta undo` command makes it just as easy to fix the mistake, but effectively reversing the most recent change.
+The `ta undo` command makes it just as easy to fix the mistake, by effectively reversing the most recent change.
 Multiple invocations of `ta undo` can be used to undo multiple changes.
 
 The limit of this functionality is that changes which have been synchronized to the server (via `ta sync`) cannot be undone.