datetime: fixed totable wday desc

See https://github.com/tarantool/tarantool/blob/c61d056cb2ed9c310ca4e54f8246a811883a7394/src/lua/datetime.lua#L977 .

Author: tmr-g

doc/reference/reference_lua/datetime.rst

@@ -806,6 +806,7 @@ datetime_object
 
                 *   -   wday
                     -   Days since the beginning of the week. Number.
+                        1 is Sunday as for ``os.date('*t')``.
 
                 *   -   yday
                     -   Days since the beginning of the year. Number.