Add package name to example Cargo.toml (#290)

This commit adds a `[package] name = "..."` entry to the example `Cargo.toml` file in the `Swift Packages` chapter of the book.

This prevents the example crate from being interpreted as a virtual manifest.
This is important because a Cargo virtual manifest cannot have a `[lib]` section.

book/src/building/swift-packages/README.md

@@ -18,6 +18,9 @@ mkdir my-rust-lib && cd my-rust-lib
 ```toml
 # Cargo.toml
 
+[package]
+name = "my-rust-lib"
+
 [lib]
 crate-type = ["staticlib"]