Add omitted word

Author: dgs3

source/core/index-creation.txt

@@ -17,8 +17,8 @@ Index Builds on Populated Collections
 
 
 Starting in MongoDB 4.2, index builds use an optimized build process
-holds an exclusive lock on the collection at the beginning and end of
-the index build. The rest of the build process yields to interleaving
+that holds an exclusive lock on the collection at the beginning and end
+of the index build. The rest of the build process yields to interleaving
 read and write operations. For a detailed description of index build
 process and locking behavior, see :ref:`index-build-process`.