add badge to codecov

Author: link-duan

README.md

@@ -3,15 +3,19 @@
 <p align="center">
   <img src="https://goreportcard.com/badge/github.com/link-duan/excel-formula-parser"/>
   <img src="https://github.com/link-duan/excel-formula-parser/actions/workflows/go.yml/badge.svg"/>
-  <img src="https://raw.githubusercontent.com/link-duan/excel-formula-parser/badges/.badges/main/coverage.svg"/>
+  <a href="https://codecov.io/gh/link-duan/excel-formula-parser" > 
+    <img src="https://codecov.io/gh/link-duan/excel-formula-parser/graph/badge.svg?token=MN0H60O7LB"/> 
+  </a>
 </p>
 
 ## Supportted syntax
+
 - [x] Math operations (eg. `+ - * / ^ %`)
 - [x] Cell references ( Absolute & Relative. eg. `$A$1` `A1:B2` )
 - [x] Function call
 
 ## Usage
+
 ```go
 ast, _ := excelformulaparser.NewParser("=SUM(A1:B2, C$3, 4:4)").Parse()
 fmt.Printf("%v", ast)