langchain

e635c861 - Slightly modified the docstring in `BasePromptTemplate` and `StringPromptTemplate`. (#1755)

Slightly modified the docstring in `BasePromptTemplate` and `StringPromptTemplate`. (#1755) Regarding [this issue](https://github.com/hwchase17/langchain/issues/1754), `BasePromptTample` class docstring is a little outdated, thus it requires new method `format_prompt` for now. As such, I have made some modifications to the docstring to bring it up to date. I tried to adhere to the established document style, and would appreciate you for taking a look at this PR.