Update join-logic.md (#5807)

making "Unique" lowercase to fit with our styling

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website/docs/docs/build/join-logic.md

@@ -122,7 +122,7 @@ semantic_models:
       - name: user_id
         type: primary
       - name: country_id
-        type: Unique
+        type: unique
     dimensions:
       - name: signup_date
         type: time