made header language typescript

[markgrahamdawson]

These changes partially address #xxxx
These changes close #xxxx
This is a small change with no associated Issue.

Requirements 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).

• Appropriate tests are included (unit and/or functional).
• Already covered by existing tests.
• Does not need tests (why?).

• Appropriate change log entry included.
• No change log entry required (why? e.g. invisible to users).

• I have opened a documentation PR at cylc/cylc-doc/pull/XXXX.
• No documentation update required.