Set license explicitly

Summary:
This CL sets the `license` attribute in `Cargo.toml` to MIT

# Why?

Clarifiy OSS use conditions. Referening license file instead of specifying `license` directly makes `crates.io` say this is a "non standard license". Setting instead `license` then also shows the license correctly.

https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields

# What?

- Removed `license-file` that pointed to the included `LICENSE` file which contains a MIT license copy
- Added `license = "MIT"`

Reviewed By: AndreasBackx

Differential Revision: D68321468

fbshipit-source-id: 2dccef335e1386d45edf16a7c0fe1784c448f83d

Cargo.toml

@@ -1,10 +1,10 @@
 [package]
 name = "scrut"
-version = "0.2.0"
+version = "0.2.1"
 edition = "2021"
 autobins = false
 description = "A simple and powerful test framework for CLI applications"
-license-file = "LICENSE"
+license = "MIT"
 repository = "https://github.com/facebookincubator/scrut"
 readme = "README.md"
 categories = [