Update multi-stage-calculations.mdx

Author: paco-valdez

docs/pages/product/data-modeling/concepts/multi-stage-calculations.mdx

@@ -242,7 +242,7 @@ For example, comparing revenue sales to the overall average:
 
 - name: revenue_per_city_sq_feet
   multi_stage: true
-  sql: "{revenue} / NULLIF({occupied_sq_feet_per_city}, 0)"
+  sql: "CASE WHEN {occupied_sq_feet_per_city} = 0 THEN NULL ELSE {revenue} / (CASE WHEN {occupied_sq_feet_per_city} = 0 THEN 0.1 ELSE {occupied_sq_feet_per_city} END) END"
   type: number
 ```
 
@@ -379,4 +379,4 @@ Query and result:
 
 [link-tesseract]: https://cube.dev/blog/introducing-next-generation-data-modeling-engine
 [ref-measures]: /product/data-modeling/concepts#measures
-[ref-dimensions]: /product/data-modeling/concepts#dimensions
+[ref-dimensions]: /product/data-modeling/concepts#dimensions