Update manage-indexes.txt

Removed unneeded 'm' character.

Author: askmrsinh

source/tutorial/manage-indexes.txt

@@ -106,7 +106,7 @@ Modify an Index
 ---------------
 
 To modify an existing index, you need to drop and recreate the index
-with the exception of m TTL indexes.  
+with the exception of TTL indexes.  
 
 If you need to rebuild indexes for a collection you can use the
 :method:`db.collection.reIndex()` method to rebuild all indexes on a