Tabs to spaces to be consistent

```
vendor/bin/indent --spaces -r framework/
vendor/bin/indent --tabs -r . --pattern=*.json
vendor/bin/indent --spaces -r . --pattern=*.md
vendor/bin/indent --spaces -r . --pattern=yii.*.js
vendor/bin/indent --spaces -r . --pattern=*.css
```

README.md

@@ -7,12 +7,12 @@ To use this extension,  simply add the following code in your application config
 
 ```php
 return [
-	//....
+    //....
-	'components' => [
+    'components' => [
-		'mail' => [
+        'mail' => [
-			'class' => 'yii\swiftmailer\Mailer',
+            'class' => 'yii\swiftmailer\Mailer',
-		],
+        ],
-	],
+    ],
 ];
 ```