cmake: fix typo in ECH config error msg

[cpu]

👋 Just a small typo I noticed while working on wiring up rustls-ffi vTLS ECH support.

[cpu]

Going to assume the CI failures are unrelated since this should be a no-op change.

[cpu]

vszakats changed the title CMakeLists: fix typo in ECH config error msg cmake: fix typo in ECH config error msg

Would you like me to amend the commit title similarly?

(P.s. thank you for all of your CMake work. It made my IDE experience a lot nicer 🚀 )

[vszakats]

Thanks, glad to hear!

It's fine as it is, my script will pick the title, message from the PR.

[vszakats]

Thanks @cpu!