[show] Use proper variable to avoid exception in natshow script (sonic-net#1383)

* Substituting nat_entry variable, instead of s inside fetch_translations
  function to avoid exception throwing inside the function due to undefined
  variable.

The issue has introduced in be63918, while migrating to Python 3.
Variable s has replaced with nat_entry.

Signed-off-by: Maksym Belei <Maksym_Belei@jabil.com>

Author: maksymbelei95

scripts/natshow

@@ -134,7 +134,7 @@ class NatShow(object):
             translated_dst = "---"
             translated_src = "---"
 
-            ent = self.asic_db.get_all('ASIC_DB', s, blocking=True)
+            ent = self.asic_db.get_all('ASIC_DB', nat_entry, blocking=True)
 
             nat_type = nat['nat_type']