Fixes opam message for Ivette.

opam

@@ -182,8 +182,8 @@ Use 'why3 config detect' to configure new provers.
 "Ivette is a new GUI for Frama-C, currently in development.
 Run 'ivette' once to finalize installation (requires an internet connection).
 Once finalized, 'ivette' will work offline.
-Finalization also requires Node v16 and Yarn:
+Finalization also requires Node v20 and Yarn:
 - install NVM (https://github.com/nvm-sh/nvm)
-- run 'nvm use 16'
+- run 'nvm use 20'
 - run 'npm install --global yarn'" { success }
 ]