docs(README.md): update codecov badge to reflect the main branch for accurate coverage reporting

Author: tumf

README.md

@@ -1,6 +1,6 @@
 # MCP Text Editor Server
 
-[![codecov](https://codecov.io/gh/tumf/mcp-text-editor/graph/badge.svg?token=52D51U0ZUR)](https://codecov.io/gh/tumf/mcp-text-editor)
+[![codecov](https://codecov.io/gh/tumf/mcp-text-editor/branch/main/graph/badge.svg?token=52D51U0ZUR)](https://codecov.io/gh/tumf/mcp-text-editor)
 
 A Model Context Protocol (MCP) server that provides line-oriented text file editing capabilities through a standardized API. Optimized for LLM tools with efficient partial file access to minimize token usage.