changed installation instructions (#905)

* changed installation instructions

* `install` added to the command

---------

Co-authored-by: Ranidu Harshana <[email protected]>
Co-authored-by: Ranidu Harshana <[email protected]>

Author: ranidu-harshana

sections/basics/installation.md

@@ -4,7 +4,7 @@ Installing styled-components only takes a single command and you're ready to rol
 
 ```
 # with npm
-npm install --save styled-components
+npm install styled-components
 
 # with yarn
 yarn add styled-components

sections/homepage/getting-started.md

@@ -28,7 +28,7 @@ export default ({ children }) => (
 To download styled-components run:
 
 ```
-npm install --save styled-components
+npm install styled-components
 ```
 
 That's all you need to do, you are now ready to use it in your app! (yep, no build step needed 👌)