fix(docs): make it clear that volume name has to start with a letter

[ahasna]

In the DigitalOcean API docs for creating a volume, it is stated that the volume must start wit a letter which is missing from the provider docs.

name required | string 

A human-readable name for the block storage volume.
Must be lowercase and be composed only of numbers, letters and "-", up to a limit of 64 characters. 
The name must begin with a letter.

[andrewsomething]

👍 Nice catch. Thanks!