[MINOR] Add example for MAP type in reference docs

site/_docs/reference.md

@@ -1200,7 +1200,7 @@ Note:
 | ANY      | The union of all types |
 | UNKNOWN  | A value of an unknown type; used as a placeholder |
 | ROW      | Row with 1 or more columns | Example: Row(f0 int null, f1 varchar)
-| MAP      | Collection of keys mapped to values |
+| MAP      | Collection of keys mapped to values | Example: MAP\['washington', 1, 'obama', 44\]
 | MULTISET | Unordered collection that may contain duplicates | Example: int multiset
 | ARRAY    | Ordered, contiguous collection that may contain duplicates | Example: varchar(10) array
 | CURSOR   | Cursor over the result of executing a query |