website: v0.15 upgrade guide had invalid example for tomap(...)

tomap expects an object value with braces, not a tuple value with brackets.

website/upgrade-guides/0-15.html.markdown

@@ -203,7 +203,7 @@ upgrading to Terraform v0.15:
     ```
 
     If you need to update a module which was using the `map` function, you
-    can get the same result by replacing `map(...)` with `tomap([...])`.
+    can get the same result by replacing `map(...)` with `tomap({...})`.
     For example:
 
     ```diff