trivial: lower subheading level

Author: kay-kim

source/core/schema-validation.txt

@@ -122,14 +122,14 @@ query expression:
 .. seealso:: :ref:`query operators <query-selectors>`
 
 Behavior
---------
+~~~~~~~~
 
 Validation occurs during updates and inserts. When you add validation to
 a collection, existing documents do not undergo validation checks until
 modification.
 
-Existing Documents
-~~~~~~~~~~~~~~~~~~
+Validation Level
+````````````````
 
 The ``validationLevel`` option determines which operations MongoDB
 applies the validation rules:
@@ -196,7 +196,7 @@ To disable validation entirely, you can set ``validationLevel`` to
 ``off``.
 
 Accept or Reject Invalid Documents
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+``````````````````````````````````
 
 The ``validationAction`` option determines how MongoDB handles
 documents that violate the validation rules:
@@ -255,15 +255,15 @@ proceed:
    2017-12-01T12:31:23.738-0500 W STORAGE  [conn1] Document would fail validation collection: example.contacts2 doc: { _id: ObjectId('5a2191ebacbbfc2bdc4dcffc'), name: "Amanda", status: "Updated" }
 
 Restrictions
-------------
+~~~~~~~~~~~~
 
 You cannot specify a validator for collections in the ``admin``,
 ``local``, and ``config`` databases.
 
 You cannot specify a validator for ``system.*`` collections.
 
 Bypass Document Validation
---------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Users can bypass document validation using the
 ``bypassDocumentValidation`` option. For a list of commands that