Update example env

[MattyTheHacker]

The example env still mentions the membership url, which is no longer used in favour of the org id

[anagha-cmd]

Can you please add some more details and assign this to me? As per my current understanding these lines in the example env (and their subsequent mention in the README) have to be modified/removed:
PURCHASE_MEMBERSHIP_URL=[Replace with your group\'s purchase-membership URL]
ORGANISATION_ID=[Replace with your group\'s MSL Organisation ID]

[MattyTheHacker]

Refer to 2bad530

Instead of using the url directly instead we take the organisation ID and construct the url within the code