Slightly modified the docstring in `BasePromptTemplate` and `StringPromptTemplate`.
I'm so sorry for my forgetting lint. 😞 
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[this issue](https://github.com/hwchase17/langchain/issues/1754), `BasePromptTample` class docstring is a little outdated, thus it requires new method `format_prompt` for now.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.