Added example with function argument expansion

Even if the expansion with three dots is explicity mentioned in https://www.terraform.io/language/expressions/function-calls#expanding-function-arguments the additional example would have helped me a lot as it is a common use case to "flatten" a list of maps.

website/docs/language/functions/merge.mdx

@@ -27,6 +27,15 @@ type structure of the attributes after the merging rules have been applied.
 }
 ```
 
+```
+> merge([{a="b", c="d"}, {}, {e="f", c="z"}]...)
+{
+  "a" = "b"
+  "c" = "z"
+  "e" = "f"
+}
+```
+
 ```
 > merge({a="b"}, {a=[1,2], c="z"}, {d=3})
 {