docs: add example for map's `.keys()` method (#11397)

doc/docs.md

@@ -1043,6 +1043,7 @@ m['two'] = 2
 println(m['one']) // "1"
 println(m['bad_key']) // "0"
 println('bad_key' in m) // Use `in` to detect whether such key exists
+println(m.keys()) // ['one', 'two']
 m.delete('two')
 ```
 Maps can have keys of type string, rune, integer, float or voidptr.