docs: nicer quotes

It's generally a good practice to prefer single-quotes in shell script strings unless you plan on interpolating environment variables.

It's purely stylistic here, but there's something about  shell code that looks safer at first-glance. It's sorta like the how SQL query string _can_ be safe when concatenating another string directly onto it, but it doesn't _look_ as safe at first glance as one that escapes the inputs.

Plus, given that the NPM script is already inside a JSON string, getting rid of the escaped characters makes it look neater! :3

Author: IsaacLeeWebDev

README.md

@@ -61,7 +61,7 @@ The tool is written in Node.js, but you can use it to run **any** commands.
 Remember to surround separate commands with quotes:
 
 ```bash
-concurrently "command1 arg" "command2 arg"
+concurrently 'command1 arg' 'command2 arg'
 ```
 
 Otherwise **concurrently** would try to run 4 separate commands:
@@ -70,7 +70,7 @@ Otherwise **concurrently** would try to run 4 separate commands:
 In package.json, escape quotes:
 
 ```bash
-"start": "concurrently \"command1 arg\" \"command2 arg\""
+"start": "concurrently 'command1 arg' 'command2 arg'"
 ```
 
 You can always check concurrently's flag list by running `concurrently --help`.