transaction: fix a comment typo

Summary: as titled

Reviewed By: lmvasquezg

Differential Revision: D91446018

fbshipit-source-id: 3b4d171c27141d3f4de53c7399cc93aac50166aa

Author: taidaii

eden/scm/sapling/transaction.py

@@ -391,7 +391,7 @@ def registertmp(self, tmpfile, location=""):
 
     @active
     def addfilegenerator(self, genid, filenames, genfunc, order=0, location=""):
-        """add a function to generates some files at transaction commit
+        """add a function to generate some files at transaction commit
 
         The `genfunc` argument is a function capable of generating proper
         content of each entry in the `filename` tuple.