Update source/tutorial/model-data-for-keyword-search.txt

fix a typo

Author: obormot

source/tutorial/model-data-for-keyword-search.txt

@@ -67,7 +67,7 @@ values from the array.
 
       db.volumes.findOne( { topics : "voyage" }, { title: 1 } )
 
-.. note:: An array with a large number many elements, such as one with
+.. note:: An array with a large number of elements, such as one with
    several hundreds or thousands of keywords will incur greater
    indexing costs on insertion.