Add missing code tag to Rules documentation (#2090)

Co-authored-by: Cal Stephens <cal@calstephens.tech>

Rules.md

@@ -3117,6 +3117,7 @@ Swift 6.1 and later:
 -     Quux
 +     Quux,
   > {}
+```
 
 </details>
 <br/>

Sources/Rules/TrailingCommas.swift

@@ -146,6 +146,7 @@ public extension FormatRule {
         -     Quux
         +     Quux,
           > {}
+        ```
         """
     }
 }