[mlir][tblgen][docs] Use correct introductionary prefix for the Syntax...

[mlir][tblgen][docs] Use correct introductionary prefix for the Syntax description of attributes and types

The doc generator currently has the use of `!` as prefix hardcoded, despite being incorrect for Attributes. These start with `#`.

This patch fixes that little issue by using `#` for AttrDefs and `!` for TypeDefs in the `Syntax` field of the generated Markdown file.

Differential Revision: https://reviews.llvm.org/D139524