Fixe docs anchor for basic SRC-7 example

Type of change

The following changes have been made:

[ ] I have linked to any relevant issues.
[ ] I have commented my code, particularly in hard-to-understand areas.
[ ] I have updated the documentation where relevant (API docs, the reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have requested support from the DevRel team
[ ] I have added tests that prove my fix is effective or that my feature works.
[ ] I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
[ ] I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
[ ] I have requested a review from the relevant team or maintainers.
[ ] I have updated the changelog to reflect the changes on this PR.