Add link to PR on Github in the tutorial

tutorial/westeros/westeros_investment.ipynb

@@ -54,7 +54,7 @@
     "    INVEST(node, year) =E= SUM(tec$( map_tec(node,tec,year) ), ( inv_cost(node,tec,year) * construction_time_factor(node,tec,year) * end_of_horizon_factor(node,tec,year) * CAP_NEW(node,tec,year) )$( inv_tec(tec) )\n",
     ");`\n",
     "\n",
-    "If you are not sure, you can check these changes here:\n",
+    "If you are not sure, you can check these changes here: https://github.com/iiasa/message_ix/pull/602\n",
     "\n",
     "At this stage, you do not need to update the model documentation as it is done in the link above. But this is a good modeling practice for documenting the new sets, parameters, variables, and equations in the GAMS files using inline Latex notes."
    ]