BUG/MINOR: http: fix misleading error message for response captures

Kay Fuchs reported that the error message is misleading in response
captures because it suggests that "len" is accepted while it's not.

This needs to be backported to 1.6.
diff --git a/src/proto_http.c b/src/proto_http.c
index 4a6e6d3..bab3db5 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -12519,7 +12519,7 @@
 			break;
 
 	if (cur_arg < *orig_arg + 3) {
-		memprintf(err, "expects <expression> [ 'len' <length> | id <idx> ]");
+		memprintf(err, "expects <expression> id <idx>");
 		return ACT_RET_PRS_ERR;
 	}
 
@@ -12537,7 +12537,7 @@
 	}
 
 	if (!args[cur_arg] || !*args[cur_arg]) {
-		memprintf(err, "expects 'len or 'id'");
+		memprintf(err, "expects 'id'");
 		free(expr);
 		return ACT_RET_PRS_ERR;
 	}