Update README.md

[apohlidal]

Goal
Fix the description of configuration when you want to use your analysis_options.yaml file with SonarQube.

What
In README.md, I changed sonar.dart.analyzer.options.override= from true to false.

Why
The adjusted part describes how to use your own analysis_options.yaml instead of the default file. There is a suggestion that this behavior could be disabled by setting an option to true. But this doesn't align with reality and with the documentation that is further below under the section Available options:

• The true value is used by default and with this set, the analysis_options.yaml file provided by this project is used.
• The false value is used when you want to use your own analysis_options.yaml file.