Made jinja2 subsitution regex less greedy in gedit syntax highlighter

[pdearnshaw]

The regex for jinja2 subsitution in gedit syntax highlighter is too greedy, for example if you have two variables on a single line then all the text between the two variables gets highlighted as jinja2 instead of just the variables. e.g.

{{VAR1}} extra text {{VAR2}}

would have the "extra text" syntax highlighted as jinja2, where as with the change only the {{VAR1}} and {{VAR2}} would be syntax highlighted as jinja2.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

Makes sense, tested in gedit, works nicely, thanks.