Fix "expected expression, found ," compile error for precompiled shaders with more than one entrypoint

[Arc-blroth]

Commit c6959aa seems to have introduced a regression that causes the vulkano-shaders macro to emit two consecutive commas for any single shader module with more than one entry point. (Since vulkano-shaders doesn't generate shader modules with multiple entrypoints itself as far as I know, this regression only manifests on precompiled shaders in practice.)

Here's a minimal reproducible example with two entrypoints vert and frag: shaders.spv.zip

mod shaders {
    vulkano_shaders::shader!{
        ty: "vertex",
        bytes: "shaders.spv",
    }
}

Dumping the generated macro output reveals the double comma of doom:

unsafe {
    ::vulkano::shader::ShaderModule::from_words_with_data(
        // -- snip --
        [
            (
                "vert".to_owned(),
                ::vulkano::shader::spirv::ExecutionModel::Vertex,
                ::vulkano::shader::EntryPointInfo {
                    // -- snip --
                },
            ),, // <-- ERROR
            (
                "frag".to_owned(),
                ::vulkano::shader::spirv::ExecutionModel::Fragment,
                ::vulkano::shader::EntryPointInfo {
                    // -- snip --
                },
            ),
        ],
    )
}

Luckily, the fix for this is to simply remove the offending comma from the write_entry_point method - a one character change that makes this PR quite possibly the shortest PR I've ever authored :D

Changelog:

### Bugs fixed
- Vulkano-shaders: Fixed an "expected expression, found `,`" compile error for precompiled shaders with more than one entrypoint.