Update ir.md (#10319)

Hello,

A possible typo in this text:

"Forward" Should be "Foreword"

"Forward" means to move ahead, but the correct term for an introduction is "Foreword".

Thanks.

Author: owenzimmew06

cranelift/docs/ir.md

@@ -1,6 +1,6 @@
 # Cranelift IR Reference
 
-## Forward
+## Foreword
 
 This document is likely to be outdated and missing some important
 information. It is recommended to look at the list of instructions as