Fix exec error in markdown

Since test_can_run_docs_code_snippets will run the snippets
in markdown, fix the undefined error in the snippets.

Author: ToastCheng

docs/dev/serialization.md

@@ -36,13 +36,13 @@ print(text)
 The JSON can also be written to a file:
 
 ```python
-cirq.to_json(obj, filepath)
+cirq.to_json(obj, '<filepath>.json')
 ```
 
 Or read back in from a file:
 
 ```python
-obj = cirq.read_json(filepath)
+obj = cirq.read_json('<filepath>.json')
 ```
 
 Or read back in from a string:

docs/dev/style.md

@@ -108,6 +108,8 @@ Note that type annotations may need to be quoted when they act in expressions
 that are evaluated at the import time, for example,
 
 ```python
+from typing import Union
+
 MOMENT_OR_OPERATION = Union['cirq.Moment', 'cirq.Operation']
 ```