Update README.md

Change travis build badge to github actions badge.

Author: exjam

README.md

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-[![Build Status](https://travis-ci.org/devkitPro/wut-tools.svg?branch=master)](https://travis-ci.org/devkitPro/wut-tools)
+[![Build status](https://github.com/devkitPro/wut-tools/workflows/C/C++%20CI/badge.svg?branch=master)](https://github.com/devkitPro/wut-tools/actions?workflow=C%2FC%2B%2B+CI)
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 # wut-tools
 Tools for [wut](https://github.com/devkitPro/wut).