Auto merge of #23167 - rprichard:fix-plugin-rustbook, r=steveklabnik

 * "let mut text" was previously of &String type.  Now it is of &str type.

 * Update the slicing syntax.  Both &text[] and text.slice_from() evaluate
   to a &str.

 * We were passing a u32 to expr_usize.  Call expr_u32 instead.

r? @steveklabnik
Fixes #23166