[Dashboard] Fix execution dashboard unit
[Dashboard] Fix execution dashboard unit

The unit should be second instead of microsecond.

Closes: #851
Approved by: fbandersnatch

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core