Add XML closing tag to ContosoCustomPolicy.XML

This commit adds a missing closing tag to the ContosoCustomPolicy.XML file, which was causing issues with parsing the XML. The closing tag was added to the end of the file to ensure proper formatting and compatibility with XML parsers

Author: CliveTran

articles/active-directory-b2c/custom-policies-series-hello-world.md

@@ -59,7 +59,7 @@ If you haven't already done so, create the following encryption keys. To automat
 
             <BuildingBlocks>
                 <!-- Building Blocks Here-->
-            <BuildingBlocks>
+            </BuildingBlocks>
 
             <ClaimsProviders>
                 <!-- Claims Providers Here-->
@@ -303,4 +303,4 @@ Next, learn:
 
 - About custom policy [claims data type](claimsschema.md#datatype). 
 
-- About custom policy [user input types](claimsschema.md#userinputtype).
+- About custom policy [user input types](claimsschema.md#userinputtype).