Updating union type error message and readme

Author: swansonk14

README.md

@@ -262,11 +262,11 @@ Literal is analagous to argparse's [choices](https://docs.python.org/3/library/a
 More complex types _must_ be specified with the `type` keyword argument in `add_argument`, as in the example below.
 
 ```python
-def to_number(string: str):
+def to_number(string: str) -> Union[float, int]:
     return float(string) if '.' in string else int(string)
 
 class MyTap(Tap):
-    number: Union[int, float]
+    number: Union[float, int]
 
     def configure(self):
         self.add_argument('--number', type=to_number)

tap/tap.py

@@ -187,11 +187,13 @@ def _add_argument(self, *name_or_flags, **kwargs) -> None:
                         raise ArgumentTypeError(
                             'For Union types, you must include an explicit type function in the configure method. '
                             'For example,\n\n'
+                            'def to_number(string: str) -> Union[float, int]:\n'
+                            '    return float(string) if \'.\' in string else int(string)\n\n'
                             'class Args(Tap):\n'
-                            '    arg: Union[int, float]\n'
+                            '    arg: Union[float, int]\n'
                             '\n'
                             '    def configure(self) -> None:\n'
-                            '        self.add_argument("--arg", type=lambda x: float(x) if "." in x else int(x))'
+                            '        self.add_argument(\'--arg\', type=to_number)'
                         )
 
                     if len(var_args) > 0: