Change license metadata (#318)

The Cargo book has a section that describes how these fields are supposed to be used:
<https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields>

It says that the `license-file` key is for the case where "a package is using a nonstandard
license". Since our license is MIT, we should use the `license` key (like almost all of the Cargo
ecosystem).

Author: jorendorff

crates/twirp-build/Cargo.toml

@@ -11,7 +11,7 @@ categories = [
     "asynchronous",
 ]
 repository = "https://github.com/github/twirp-rs"
-license-file = "./LICENSE"
+license = "MIT"
 
 [dependencies]
 prost-build = "0.14"

crates/twirp/Cargo.toml

@@ -11,7 +11,7 @@ categories = [
     "asynchronous",
 ]
 repository = "https://github.com/github/twirp-rs"
-license-file = "./LICENSE"
+license = "MIT"
 
 [features]
 test-support = []