docs: update coverage badge in README

- Updated coverage percentage

Author: fulleni

README.md

@@ -1,6 +1,6 @@
 # ht_api
 
-![coverage: percentage](https://img.shields.io/badge/coverage-XX-green)
+![coverage: percentage](https://img.shields.io/badge/coverage-54-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)