Update license link in README.md (bevyengine#5614)

# Objective

The `LICENSE` file in the root directory was removed in bevyengine#4966. This breaks the license shield link in the README.

## Solution

I changed the link to instead point at the license section of the readme on the main repo page. I think this is better than a 404, but I am unsure as to if it's the best solution. As such feedback is appreciated.

README.md

@@ -1,7 +1,7 @@
 # [![Bevy](assets/branding/bevy_logo_light_dark_and_dimmed.svg)](https://bevyengine.org)
 
 [![Crates.io](https://img.shields.io/crates/v/bevy.svg)](https://crates.io/crates/bevy)
-[![MIT/Apache 2.0](https://img.shields.io/badge/license-MIT%2FApache-blue.svg)](./LICENSE)
+[![MIT/Apache 2.0](https://img.shields.io/badge/license-MIT%2FApache-blue.svg)](https://github.com/bevyengine/bevy#license)
 [![Crates.io](https://img.shields.io/crates/d/bevy.svg)](https://crates.io/crates/bevy)
 [![Rust](https://github.com/bevyengine/bevy/workflows/CI/badge.svg)](https://github.com/bevyengine/bevy/actions)
 ![iOS cron CI](https://github.com/bevyengine/bevy/workflows/iOS%20cron%20CI/badge.svg)