update github action names

Author: rainliu

.github/workflows/cargo.yml

@@ -1,4 +1,4 @@
-name: Cargo
+name: cargo
 
 on:
   push:

.github/workflows/grcov.yml

@@ -1,4 +1,4 @@
-name: Coverage
+name: coverage
 
 on:
   push:

README.md

@@ -4,7 +4,7 @@
 </h1>
 <p align="center">
  <a href="https://github.com/webrtc-rs/rtcp/actions"> 
-  <img src="https://github.com/webrtc-rs/rtcp/workflows/Cargo/badge.svg">
+  <img src="https://github.com/webrtc-rs/rtcp/workflows/cargo/badge.svg">
  </a> 
  <a href="https://codecov.io/gh/webrtc-rs/rtcp"> 
   <img src="https://codecov.io/gh/webrtc-rs/rtcp/branch/main/graph/badge.svg">