Updates branch in Test Coverage badge to main

README.md

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 [![Go Report Card](https://goreportcard.com/badge/github.com/auyer/steganography)](https://goreportcard.com/report/github.com/auyer/steganography)
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 [![Mentioned in Awesome Go](https://awesome.re/mentioned-badge.svg)](https://github.com/avelino/awesome-go) 
 
 Steganography is a library written in Pure go to allow simple LSB steganography on images. It is capable of both encoding and decoding images. It can store files of any format.