Fix "#" being converted to "::" in doxygen

Author: Owen Jones

doc/architectural/data-structures-core-structures-and-ast.md

@@ -27,8 +27,8 @@ See documentation at \ref irept.
 As that documentation says, `irept`s are generic tree nodes. You should
 think of them as having a single string (data, actually an `irep_idt`) and
 lots of child nodes, some of which are numbered (sub) and some of which are
-labelled, and the label can either start with a “#” (comments-sub) or without
-one (named-sub). The meaning of the “#” is that this child should not be
+labelled, and the label can either start with a “\#” (comments-sub) or without
+one (named-sub). The meaning of the “\#” is that this child should not be
 considered important, for example it shouldn’t be counted when comparing two
 `irept`s for equality.