Update VE Swap scenario

[AnshuJalan]

Update the scenario mentioned at: https://github.com/Plenty-DeFi/ve-core/blob/master/docs/Scenarios.md#1-swap-existing-plenty-or-wrap-tokens-with-ply-or-associated-veply-tokens

Current mechanism:

• For an exchange rate of 5 PLY / PLENTY
• If a user swaps 100 PLENTY (500 PLY to be received in total)
• The user receives 250 PLY (50%) immediately, and remaining 250 PLY is vested linearly over a period (1 month is testnet contracts).
• The user can make claim of vested tokens once every 24 hours.
• The vesting is flexible -
  • If a user swaps 100 PLENTY after the END timestamp, they end up getting the whole sum of 500 PLY at once.
  • If a user swaps 100 PLENTY at the end of one year, they get 250 PLY + 100 PLY (a sum that would have been claimable, had the user swapped at genesis).