[otbn] fix typo in modexp.s

Fix a typo that slipped in #3332

Signed-off-by: Felix Miller <felix.miller@gi-de.com>
diff --git a/sw/otbn/code-snippets/modexp.S b/sw/otbn/code-snippets/modexp.S
index 8e9883d..cb1e6dd 100644
--- a/sw/otbn/code-snippets/modexp.S
+++ b/sw/otbn/code-snippets/modexp.S
@@ -336,7 +336,7 @@
 ECALL
 
 sqrx_exp:
-LW x16, 24(x0)
+LW x16, 32(x0)
 LW x17, 36(x0)
 LW x18, 40(x0)
 LW x19, 44(x0)