Removed reference to 'echo-scenario'

[explodingdinosaurs]

echo-scenario doesn't exist. It was a plan at one point, but was supplanted by nueva. nueva doesn't use sgt so the reference will be removed.

[mikejturner]

✅ Code coverage on remove_echo_scenario_in_readme is at 81% (compared to main being at 81%)

🎉 No files have reduced coverage 🎉

Full Coverage Report

File	Details
src/echo/__init__.py	✅ 100% → 100%
src/echo/configuration.py	✅ 100% → 100%
src/echo/constants.py	✅ 100% → 100%
src/echo/exceptions.py	✅ 88% → 88%
src/echo/functional.py	✅ 100% → 100%
src/echo/models/__init__.py	✅ 100% → 100%
src/echo/models/agnostic.py	✅ 86% → 86%
src/echo/models/base.py	✅ 82% → 82%
src/echo/models/carbon.py	✅ 100% → 100%
src/echo/models/electrical.py	✅ 74% → 74%
src/echo/models/gas.py	✅ 72% → 72%
src/echo/models/prebuilt.py	✅ 56% → 56%
src/echo/models/scenario.py	✅ 100% → 100%
src/echo/models/thermal.py	✅ 81% → 81%
src/echo/objectives/__init__.py	✅ 100% → 100%
src/echo/objectives/base.py	✅ 98% → 98%
src/echo/objectives/contingency.py	✅ 93% → 93%
src/echo/objectives/power.py	✅ 86% → 86%
src/echo/objectives/tariff.py	✅ 79% → 79%
src/echo/optimiser.py	✅ 77% → 77%
src/echo/utils.py	✅ 59% → 59%
src/echo/validators.py	✅ 88% → 88%
TOTAL	✅ 81% → 81%