Use ignore instead of text (rp-rs#378)

While the code block fails to compile, it's still code and not random text. Marking it with `ignore` allows for proper syntax highlighting, for example.

According to rust-lang/rust#97030 (comment) this is the proper fix, so this closes rp-rs#374

rp2040-hal/src/intrinsics.rs

@@ -58,7 +58,7 @@ macro_rules! intrinsics_aliases {
 /// Like the compiler-builtins macro, it accepts a series of functions that
 /// looks like normal Rust code:
 ///
-/// ```text
+/// ```ignore
 /// intrinsics! {
 ///     extern "C" fn foo(a: i32) -> u32 {
 ///         // ...