Remove Known Issue of Full Screen mode

[faisal-alvi]

Description of the Change

As we have already fixed the full-screen issue (in #750) mentioned in the readme file. We should remove it.

distributor/README.md

Line 128 in 9c90725

__Gutenberg Fullscreen Mode__ - [Gutenberg 3.8](https://wptavern.com/gutenberg-3-8-released-adds-full-screen-mode) originally introduced `Fullscreen mode` for the editor and [WordPress 5.4](https://make.wordpress.org/core/2020/03/03/fullscreen-mode-enabled-by-default-in-the-editor/) and [Gutenberg 7.7](https://github.com/WordPress/gutenberg/pull/20611) made that the default setting. Fullscreen mode creates a problem as the admin bar is no longer visible which means the Distributor push menu is no longer visible. We are [working on researching a resolution to this issue](https://github.com/10up/distributor/issues/597), but in the meantime we recommend clicking on the three vertical dots in the upper right corner of Gutenberg and disabling fullscreen mode to ensure the admin bar and Distributor push menu is in view.

Alternate Designs

n/a

Possible Drawbacks

n/a

[jeffpaul]

Great catch on this @faisal-alvi, thank you!