Update migrating-from-v3 README typo (#4746)

Changed `--sav-dev` to `--save-dev`

Author: seangwright

docs/migrating-from-v3/README.md

@@ -224,7 +224,7 @@ It now requires `eslint`, `eslint-plugin-prettier` and `prettier` as peer depend
 For older projects, if you encountered issues like `Cannot find module: eslint-plugin-prettier`, please run the following command to fix it:
 
 ```sh
-npm install --sav-dev eslint@5 @vue/eslint-config-prettier@5 eslint-plugin-prettier prettier
+npm install --save-dev eslint@5 @vue/eslint-config-prettier@5 eslint-plugin-prettier prettier
 ```
 
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