docs: remove unnecessary args from the pip install (#19823)
**Description:** An additional `U` argument was added for the
instructions to install the pip packages for the MediaWiki Dump Document
loader which was leading to error in installing the package. Removing
the argument fixed the command to install.
**Issue:** #19820
**Dependencies:** No dependency change requierd
**Twitter handle:** [@vardhaman722](https://twitter.com/vardhaman722)
