[docs] Fix Darc.md `delete-subscriptions` typo

[JanProvaznik]

Darc docs did not correspond to reality due to a typo.

PS C:\Users\janprovaznik\dev\arcade> darc 
Microsoft.DotNet 1.1.0-beta.24510.3+c5a518175e3338198615ce1cef96f3f152daa5ad
c Microsoft Corporation. All rights reserved.
ERROR(S):
No verb selected.
...

  delete-subscriptions         Delete a subscription or set of subscriptions matching criteria.
...