Small typo in `conversation.py` docstring.
## Reproduction

There is a small typographical error in the docstring for Conversation class in conversational.py.

```python

class Conversation:
    """
    Utility class containing a conversation and its history. This class is meant to be used as an input to the
    [`ConversationalPipeline`]. The conversation contains a number of utility function to manage the addition of new
    user input and generated model responses. A conversation needs to contain an unprocessed user input before being
    passed to the [`ConversationalPipeline`]. This user input is either created when the class is instantiated, or by
    calling `conversational_pipeline.append_response("input")` after a conversation turn.

...
```

## Proposed Solution
This could be fixed by just rewriting this to:

```python
class Conversation:
    """
    Utility class containing a conversation and its history. This class is meant to be used as an input to the
    [`ConversationalPipeline`]. The conversation contains several utility functions to manage the addition of new
    user inputs and generated model responses. A conversation needs to contain an unprocessed user input before being
    passed to the [`ConversationalPipeline`]. This user input is either created when the class is instantiated, or by
    calling `conversational_pipeline.append_response("input")` after a conversation turn.

...
```


@oscar-defelice Good spot! Would you like to open a PR with the amendment? <|||||>Yes, I can open a PR and add a fixing commit.

thank you very much!