patch 9.1.0512: Mode message for spell completion doesn't match allowed keys

Problem:  Mode message for spell completion doesn't match allowed keys
          (Kyle Kovacs)
Solution: Show "^S" instead of "s".
          (zeertzjq)

This matches the code in vim_is_ctrl_x_key():

	case CTRL_X_SPELL:
	    return (c == Ctrl_S || c == Ctrl_P || c == Ctrl_N);

fixes: neovim/neovim#29431
closes: #15065

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/insexpand.c b/src/insexpand.c
index 78fea51..01fa6de 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -58,7 +58,7 @@
     N_(" Command-line completion (^V^N^P)"),
     N_(" User defined completion (^U^N^P)"),
     N_(" Omni completion (^O^N^P)"),
-    N_(" Spelling suggestion (s^N^P)"),
+    N_(" Spelling suggestion (^S^N^P)"),
     N_(" Keyword Local completion (^N^P)"),
     NULL,   // CTRL_X_EVAL doesn't use msg.
     N_(" Command-line completion (^V^N^P)"),