Fix escape character notation in RstCheatSheet

[franzholz]

Is the backslash working by this change?

[ineswillenbrock]

Looks good to me

[franzholz]

This does not work!

Source:

Escape characters
=================

If you want to use a character, which would create some special reST markup,
with its normal meaning, you must escape it with a prepended :rst:`\`.

The resulting document shows:

must escape it with a prepended :rst:``.

[ineswillenbrock]

Can confirm it while rendering locally. The first change at least rendered "\" while the proposed change is not working with the recent image, too.

edit: might it be, that packages/typo3-docs-theme/src/TextRoles/RestructuredTextTextRole.php line 24 is responsible?