PromptTemplate documentation mentions renamed prompt_text parameter

[julian-risch]

PR #4879 renamed PromptTemplate input parameters but the documentation page of PromptNode shows an example with the old parameter name. We need to update the example.

• https://docs.haystack.deepset.ai/v1.18-unstable/docs/prompt_node

We need to rename prompt_text to prompt and name="lfqa", can be removed completely in the example: https://docs.haystack.deepset.ai/v1.18-unstable/docs/prompt_node#long-form-question-answering
Here is the new implementation of PromptTemplate:
https://github.com/deepset-ai/haystack/blob/main/haystack/nodes/prompt/prompt_template.py#L367

https://docs.haystack.deepset.ai/v1.18-unstable/docs/prompt_node#prompttemplate-structure needs to be updated accordingly too: We need to rename prompt_text to prompt and name="...", can be removed completely

[dfokina]

Done, doc cleaned up :)