Update composer install command to specify non-default 3.x for 3.x README.md

Complementary to CodeWithDennis#186

Author: gp-lnuff

README.md

@@ -15,7 +15,7 @@ building selection dropdowns with various customization options.
 You can install the package via composer:
 
 ```bash
-composer require codewithdennis/filament-select-tree
+composer require codewithdennis/filament-select-tree:3.x
 ```
 
 ```bash