Fix broken link in QUICKSTART.md

[sazzy4o]

What this PR does / why we need it:

Fixes link to WRITABLE.md (link broken)

Please check the following list:

• [x] Does the affected code have corresponding tests, e.g. unit test, E2E test?
• [x] Does this change require a documentation update?
• [ ] Does this introduce breaking changes that would require an announcement or bumping the major version?
• [x] Do all new files have an appropriate license header?