Update downloading-photon.md

[Decimal6355]

update download link to V5

[vmwclabot]

@Decimal6355, you must sign our contributor license agreement before your changes are merged. Click here to sign the agreement. If you are a VMware employee, read this for further instruction.

[vmwclabot]

@Decimal6355, we have received your signed contributor license agreement. The review is usually completed within a week, but may take longer under certain circumstances. Another comment will be added to the pull request to notify you when the merge can proceed.

[vmwclabot]

@Decimal6355, VMware has rejected your signed contributor license agreement. The merge can not proceed until the agreement has been resigned. Click here to resign the agreement. Reject reason:

Minimum required personal information is Name (First and Last) and Address (Country, State, Town/City). Invalid telephone number too.