Cite legislation.gov.uk for benefit cap (#326) (#1649)

Adds SI 2016/909 reg. 2 — the amending regulations that set the
benefit cap values the parameter file currently uses from
2016-11-07. The existing UC Regulations 2013 reg. 80A reference is
kept; 80A is the application logic that the 2016 SI inserted.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vahid-ahmadi

changelog.d/326.md

@@ -0,0 +1 @@
+- Cite SI 2016/909 reg. 2 on the benefit cap parameter, which is where the 2016-11-07 values originate.

policyengine_uk/parameters/gov/dwp/benefit_cap.yaml

@@ -38,6 +38,8 @@ non_single:
       2024-04-01: 22_020
       2025-04-01: 22_020
 metadata:
-  reference: 
+  reference:
+    - title: The Benefit Cap (Housing Benefit and Universal Credit) (Amendment) Regulations 2016 reg. 2
+      href: https://www.legislation.gov.uk/uksi/2016/909/regulation/2/made
     - title: The Universal Credit Regulations 2013 reg. 80A
       href: https://www.legislation.gov.uk/uksi/2013/376/regulation/80A