ConversionToLLVMDialect doc: update the syntax for LLVM types

The syntax for LLVM dialect types changed twice since this document was
introduced. First, the quoted types are only prefixed with the dialect name
`!llvm` rather than with `!llvm.type`. Second, for types that are simple enough
(e.g., MLIR identifiers), the pretty form can be used instead of the quoted
form. The relevant commits updated the dialect documentation, but not the
conversion documentation. Use the valid type names in the conversion
documentation.

PiperOrigin-RevId: 286026153