Explicitly specify license in Cargo.toml (#23)

In the [docs for the Cargo manifest
format](https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields),
the use of `license-file` is only recommended for non-standard licenses.
For standard licenses such as MIT, it should be specified in the
`license` field explicitly.

This change makes it easier for tools to correctly identify the license.
For example, right now the license is identified as `non-standard` on
both [crates.io](https://crates.io/crates/bevy_silk) and the [Bevy
Assets page](https://bevyengine.org/assets/#physics).

Cargo.toml

@@ -4,7 +4,7 @@ version = "0.7.0"
 edition = "2021"
 authors = ["Felix de Maneville <felix.maneville@gmail.com>"]
 repository = "https://github.com/ManevilleF/bevy_silk"
-license-file = "./LICENSE"
+license = "MIT"
 exclude = ["docs", "assets", ".github"]
 description = "Cloth physics implementation in bevy"
 keywords = ["verlet", "physics", "bevy", "cloth"]