DOCSP-35045 Fix typo (#30)

Author: pierwill

source/tutorial/prepare-for-maintenance.txt

@@ -79,7 +79,7 @@ Unique Index Builds
 
 {+aagent+} builds indexes on cluster nodes one at a time using identical 
 but independent commands. To ensure that writes respect the ``unique`` 
-quality of indexed fields in a :manual:`unique indexe </core/index-unique/>`, 
+quality of indexed fields in a :manual:`unique index </core/index-unique/>`,
 all writes to the collection on the cluster must stop before you build 
 the index.