fix: correct VaeImageProcessor.blur() docstring

[sergioperezcheco]

What does this PR do?

Fixes the VaeImageProcessor.blur() docstring which had copy-pasted text from convert_to_grayscale().

Changes

• Add missing blur_factor parameter to the Args section (was entirely absent from docstring)
• Fix image arg description: was "The PIL image to convert to grayscale" → now "The input image to blur"
• Fix Returns description: was "The grayscale-converted PIL image" → now "The blurred PIL image"

Before

def blur(image: PIL.Image.Image, blur_factor: int = 4) -> PIL.Image.Image:
    r"""
    Applies Gaussian blur to an image.

    Args:
        image (`PIL.Image.Image`):
            The PIL image to convert to grayscale.

    Returns:
        `PIL.Image.Image`:
            The grayscale-converted PIL image.
    """

After

def blur(image: PIL.Image.Image, blur_factor: int = 4) -> PIL.Image.Image:
    r"""
    Applies Gaussian blur to an image.

    Args:
        image (`PIL.Image.Image`):
            The input image to blur.
        blur_factor (`int`, *optional*, defaults to 4):
            The blur factor to apply. Higher values result in more blur.

    Returns:
        `PIL.Image.Image`:
            The blurred PIL image.
    """

Fixes #14209