MIRLangRef: Simplify/update documentation

- Refer to options by `-option` instead of `option`
- Use `-mtriple=` instead of `-march` in the example (-march will still
  target the default operating system which is usually not what you want
  in a test)
- Rephrase sentence because output does not go to stdout by default (you
  need -o - for that as should be expected).

llvm-svn: 300268