DOCSP-22174 typo under partial indexes comparison (#1313)

* DOCSP-22174 typo under partial indexes comparison

* DOCSP-22174 reupdated phrasing

Author: ltran-mdb

source/core/index-partial.txt

@@ -101,7 +101,7 @@ index would return an incomplete result set.
 Comparison with Sparse Indexes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Partial indexes are be preferred over :ref:`sparse indexes
+Partial indexes should be preferred over :ref:`sparse indexes
 <index-type-sparse>`. Partial indexes provide the following benefits:
 
 - Greater control over which documents are indexed.