Merge pull request #2120 from kerneljake/patch-16

Update build-indexes-on-replica-sets.txt

Author: mpaik

source/tutorial/build-indexes-on-replica-sets.txt

@@ -152,7 +152,7 @@ To build an index on the primary you can either:
 
    #. :ref:`tutorial-index-on-replica-sets-restart-mongod`
 
-Building the index on the background, takes longer than the foreground
+Building the index in the background takes longer than the foreground
 index build and results in a less compact index structure. Additionally,
 the background index build may impact write performance on the
 primary. However, building the index in the background allows the set to