[LangRef] Remove mention of argmemonly (NFC)

Replace argmemonly readonly with memory(argmem: read).

Author: nikic

llvm/docs/LangRef.rst

@@ -27935,7 +27935,7 @@ Syntax:
 
 ::
 
-      declare {ptr, i1} @llvm.type.checked.load.relative(ptr %ptr, i32 %offset, metadata %type) argmemonly nounwind readonly
+      declare {ptr, i1} @llvm.type.checked.load.relative(ptr %ptr, i32 %offset, metadata %type) nounwind memory(argmem: read)
 
 Overview:
 """""""""