(MAINT) Fix assertion example

Prior to this change, the example configuration document for
assertions was invalid, using the `_ensure` property instead
of `_exist`. This change updates the document to be valid.

dsc/examples/assertion.dsc.yaml

@@ -5,7 +5,7 @@ resources:
   type: Microsoft.Windows/Registry
   properties:
     keyPath: HKCU\example
-    _ensure: Present
+    _exist: true
   dependsOn:
   - "[resourceId('Microsoft.DSC/Assertion','my assertions')]"
 - name: my assertions