docs(README): update code coverage shield

- Change coverage percentage from 95% to 91%
- Adjust shield color to reflect new coverage level

Author: fulleni

README.md

@@ -1,6 +1,6 @@
 # data_mongodb
 
-![coverage: 95%](https://img.shields.io/badge/coverage-95-green)
+![coverage: xx%](https://img.shields.io/badge/coverage-91-green)
 [![style: very good analysis](https://img.shields.io/badge/style-very_good_analysis-B22C89.svg)](https://pub.dev/packages/very_good_analysis)
 [![License: PolyForm Free Trial](https://img.shields.io/badge/License-PolyForm%20Free%20Trial-blue)](https://polyformproject.org/licenses/free-trial/1.0.0)