Example from docs doesn't validate

[jrmuizel]

extern crate rspirv;
extern crate spirv_headers as spirv;

use rspirv::binary::Assemble;
use rspirv::binary::Disassemble;

fn main() {
    // Building
    let mut b = rspirv::mr::Builder::new();
    b.memory_model(spirv::AddressingModel::Logical, spirv::MemoryModel::GLSL450);
    let void = b.type_void();
    let voidf = b.type_function(void, vec![void]);
    b.begin_function(void,
                     None,
                     (spirv::FunctionControl::DONT_INLINE |
                      spirv::FunctionControl::CONST),
                     voidf)
     .unwrap();
    b.begin_basic_block(None).unwrap();
    b.ret().unwrap();
    b.end_function().unwrap();
    let module = b.module();

    // Assembling
    let code = module.assemble();
    assert!(code.len() > 20);  // Module header contains 5 words
    assert_eq!(spirv::MAGIC_NUMBER, code[0]);

    // Parsing
    let mut loader = rspirv::mr::Loader::new();
    rspirv::binary::parse_words(&code, &mut loader).unwrap();
    let module = loader.module();
    use std::io::Write;
    let mut f = std::fs::File::create("out.frag").unwrap();
    let len = code[..].len() * 4;
    let buf = unsafe { std::slice::from_raw_parts(code[..].as_ptr() as *const u8, len) };
    f.write(buf);
}

gives the following error when spirv-val out.frag is run.

error: line 1: Operand 2 of MemoryModel requires one of these capabilities: Shader
  OpMemoryModel Logical GLSL450

[jrmuizel]

It seems like adding b.capability(spirv::Capability::Shader); should fix this.