Rename main branch about owasp-java-encoder

[jeremylong]

The master branch has been renamed to main. Anyone coding against master needs to update. You can either re-clone the repo or follow these instructions:

https://www.git-tower.com/learn/git/faq/git-rename-master-to-main/#what-your-teammates-have-to-do

# Switch to the "master" branch:
$ git checkout master

# Rename it to "main":
$ git branch -m master main

# Get the latest commits (and branches!) from the remote:
$ git fetch

# Remove the existing tracking connection with "origin/master":
$ git branch --unset-upstream

# Create a new tracking connection with the new "origin/main" branch:
$ git branch -u origin/main