Minor typo in pipeline FillMaskPipeline's documentation.
# What does this PR do?

Fixes a minor typo in the documentation for FillMaskPipeline.__call__().

## Before submitting
- [X ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?
@Narsil. @sgugger, @stevhliu and @MKhalusova
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you for spotting and fixing this! 