Change laravel version to 6.x

Laravel versions starting from 6 are all  6.x

Author: divine

README.md

@@ -63,7 +63,7 @@ Make sure you have the MongoDB PHP driver installed. You can find installation i
  5.6.x    | 3.4.x
  5.7.x    | 3.4.x
  5.8.x    | 3.5.x
- 6.0.x    | 3.6.x
+ 6.x      | 3.6.x
 
 Install the package via Composer: