Missing comma in example

Example https://github.com/rust-native-ui/libui-rs:
```toml
iui = { git = "https://github.com/rust-native-ui/libui-rs.git" branch="trunk" }
```
Require:
```toml
iui = { git = "https://github.com/rust-native-ui/libui-rs.git", branch="trunk" }
```