fix invalid markdown in shader docs
//! to specify one or more standard include directories. Relative paths are relative to the
//! directory, which contains the source file the `#include "..."` directive is declared in.
//!
-//! ## `define: [("NAME", "VALUE"), ...]
+//! ## `define: [("NAME", "VALUE"), ...]`
//!
//! Adds the given macro definitions to the pre-processor. This is equivalent to passing `-DNAME=VALUE`
//! on the command line.
