patch 8.2.1918: Vim9: E1100 mentions :let

Problem:    Vim9: E1100 mentions :let.
Solution:   Mention "var". (closes #7207)
diff --git a/src/vim9script.c b/src/vim9script.c
index cb13f3c..3780356 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -70,7 +70,7 @@
 	    case CMD_insert:
 	    case CMD_t:
 	    case CMD_xit:
-		semsg(_(e_missing_let_str), eap->cmd);
+		semsg(_(e_missing_var_str), eap->cmd);
 		return FAIL;
 	    default: break;
 	}