test/offlineThreshold: fix index in show variables;

tests/pytest/functions/showOfflineThresholdIs864000.py

@@ -25,7 +25,7 @@ class TDTestCase:
 
     def run(self):
         tdSql.query("show variables")
-        tdSql.checkData(51, 1, 864000)
+        tdSql.checkData(52, 1, 864000)
 
     def stop(self):
         tdSql.close()
@@ -33,4 +33,4 @@ class TDTestCase:
 
 
 tdCases.addWindows(__file__, TDTestCase())
-tdCases.addLinux(__file__, TDTestCase())
\ No newline at end of file
+tdCases.addLinux(__file__, TDTestCase())