build_table_schema references non-existent _as_json_table_type
#### Problem description
Documentation issue: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.io.json.build_table_schema.html
(and master branch contains this as well)

Description contains: 
```
See _as_json_table_type for conversion types. 
```
Which I cannot find anywhere else in the documentation

