Update Yarn README, whereisreason doesn't exist anymore

Same as #5, but for Yarn's instructions.

Author: anmonteiro

README-Yarn.md

@@ -122,7 +122,7 @@ We will come up with a long term solution at some point.
   compiled into.
 
 ```
-npm run whereisreason
+npm run whereisocamlmerlin
 ```