fix: typo (#384)

src/idioms/on-stack-dyn-dispatch.md

@@ -19,7 +19,7 @@ use std::fs;
 // These must live longer than `readable`, and thus are declared first:
 let (mut stdin_read, mut file_read);
 
-// We need to ascribe the type to get dynamic dispatch.
+// We need to describe the type to get dynamic dispatch.
 let readable: &mut dyn io::Read = if arg == "-" {
     stdin_read = io::stdin();
     &mut stdin_read