Revise docs and formatting for accounts.ex

[gmile]

In this PR I normalize the docs formatting and spelling, particularly:

• use 4 spaces for code examples,
• use lowercase letters when referencing API entity, e.g. account instead of Account,
• making note about using custom Stripe API key consistent, e.g. use "Accepts Stripe API key.
  " everywhere,
• in examples depicting usage of custom Stripe API key – use a string "my_key" instead of key variable – this is to better indicate that library users must provide their own custom string.

[gmile]

I'm using 4 spaces, as this seems to preferred way of styling the docs (per examples from Elixir codebase).

[coveralls]

Coverage remained the same at 77.778% when pulling 54968b9 on gmile:revise-docs-for-account into 782e505 on robconery:master.

[coveralls]

Coverage remained the same at 77.778% when pulling 64e4b21 on gmile:revise-docs-for-account into 782e505 on robconery:master.