Fix Document & Expose StringPromptTemplate as a custom-prompt-template.
Regarding [this issue](https://github.com/hwchase17/langchain/issues/1754), the code in the document [Creating a custom prompt template](https://langchain.readthedocs.io/en/latest/modules/prompts/examples/custom_prompt_template.html) is no longer functional and outdated. 

To address this, I have made the following changes:

1. Updated the guide in the document to use `StringPromptTemplate` instead of `BasePromptTemplate`.
2. Exposed `StringPromptTemplate` in `prompts/__init__.py` for easier importing.