Update examples to fix execution problems

[amosjyng]

On the Getting Started page for prompt templates, I believe the very last example

print(dynamic_prompt.format(adjective=long_string))

should actually be

print(dynamic_prompt.format(input=long_string))

The existing example produces KeyError: 'input' as expected

---

On the Create a custom prompt template page, I believe the line

Function Name: {kwargs["function_name"]}

should actually be

Function Name: {kwargs["function_name"].__name__}

The existing example produces the prompt:

        Given the function name and source code, generate an English language explanation of the function.
        Function Name: <function get_source_code at 0x7f907bc0e0e0>
        Source Code:
        def get_source_code(function_name):
    # Get the source code of the function
    return inspect.getsource(function_name)

        Explanation:

---

On the Example Selectors page, the first example does not define example_prompt, which is also subtly different from previous example prompts used. For user convenience, I suggest including

example_prompt = PromptTemplate(
    input_variables=["input", "output"],
    template="Input: {input}\nOutput: {output}",
)

in the code to be copy-pasted

[hwchase17]

thank you!