Fix minor grammar issue.

juliusv · juliusv · commit 2f5e6ccd19d7 · 2015-10-12T03:25:51.000+02:00
diff --git a/README.md b/README.md
@@ -113,7 +113,7 @@ with g.track_inprogress():
   pass
 ```
 
-A Gauge can also take it's value from a callback:
+A Gauge can also take its value from a callback:
 
 ```python
 d = Gauge('data_objects', 'Number of objects')
