docs: use --save-dev instead of -D

Expanded -D into the long option, so that those who aren't as familiar with the ecosystem don't have to look it up.

README.md

@@ -8,7 +8,7 @@ Husky improves your commits and more 🐶 *woof!*
 # Install
 
 ```
-npm install husky -D
+npm install husky --save-dev
 ```
 
 # Usage