Add Bash syntax highlighting to README.md

Verified exhaustiveness thanks to:

```bash
grep -r '```'
```

Author: Benjamin-Loison

README.md

@@ -31,7 +31,7 @@ If you're using PHP_CodeSniffer as part of a team, or you're running it on a [CI
 ## Installation
 
 The easiest way to get started with PHP_CodeSniffer is to download the Phar files for each of the commands:
-```
+```bash
 # Download using curl
 curl -OL https://phars.phpcodesniffer.com/phpcs.phar
 curl -OL https://phars.phpcodesniffer.com/phpcbf.phar