fix mmapv1 typo

Author: jeff-allen-mongo

source/reference/operator/aggregation/sample.txt

@@ -44,12 +44,12 @@ collection scan followed by a random sort to select N documents. In
 this case, the :pipeline:`$sample` stage is subject to the
 :ref:`sort memory restrictions <sort-memory-limit>`.
 
-MMAPV1 May Return Duplicate Documents
+MMAPv1 May Return Duplicate Documents
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 If you are using the:
 
-- MMAPV1 storage engine, :pipeline:`$sample` may return the same
+- MMAPv1 storage engine, :pipeline:`$sample` may return the same
   document more than once in the result set.
 
 - :ref:`WiredTiger <storage-wiredtiger>` or