reStructuredText: Fix inline literals highlighting preceding characters
Test Plan: Covered by automated tests, and tested locally (new sytanx file with stable Kate)
Reviewers: mosra, dhaumann
Reviewed By: dhaumann
Subscribers: nibags, dhaumann, kwrite-devel, kde-frameworks-devel
Tags: Kate, Frameworks
Differential Revision: https://phabricator.kde.org/D25276