BUG/MINOR: check: fix tcpcheck error message

add the keyword 'string' when required (error in a tcpcheck expect
string)
(cherry picked from commit 96a5c9b57738c05ecce7822093b9c4118123dc1e)
diff --git a/src/checks.c b/src/checks.c
index 71debb6..8b53f97 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -614,7 +614,7 @@
 			}
 			else if (check->last_started_step && check->last_started_step->action == TCPCHK_ACT_EXPECT) {
 				if (check->last_started_step->string)
-					chunk_appendf(chk, " (string '%s')", check->last_started_step->string);
+					chunk_appendf(chk, " (expect string '%s')", check->last_started_step->string);
 				else if (check->last_started_step->expect_regex)
 					chunk_appendf(chk, " (expect regex)");
 			}