lua: fix an argument validation error

According to the documentation, strings.from_charset expects two
arguments, not three.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/lua/libs/strings.c

@@ -125,7 +125,7 @@ static int vlclua_convert_xml_special_chars( lua_State *L )
 
 static int vlclua_from_charset( lua_State *L )
 {
-    if( lua_gettop( L ) < 3 ) return vlclua_error( L );
+    if( lua_gettop( L ) < 2 ) return vlclua_error( L );
 
     size_t i_in_bytes;
     const char *psz_input = luaL_checklstring( L, 2, &i_in_bytes );