Command Line Toolsをアップデートする

##0. 前書き
環境：macOS 10.15.2
参考：https://stackoverflow.com/a/53364628

####なぜ行ったか
Homebrewインストール後にWarningが表示された。

$ brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: A newer Command Line Tools release is available.
Update them from Software Update in System Preferences or
https://developer.apple.com/download/more/.

##1. アップデート可能かどうか調べる

$ softwareupdate --list
Software Update Tool

Finding available software
Software Update found the following new or updated software:
* Label: Command Line Tools for Xcode-11.3
	Title: Command Line Tools for Xcode, Version: 11.3, Size: 224878K, Recommended: YES,

##2. アップデートを実行する
* Label: <name> → softwareupdate -i "<name>"

$ softwareupdate -i "Command Line Tools for Xcode-11.3"
Downloading Command Line Tools for Xcode
Downloaded Command Line Tools for Xcode
Installing Command Line Tools for Xcode
Done with Command Line Tools for Xcode
Done.

##4. 実行後

$ brew doctor
Your system is ready to brew.