Leftover wording and typos in API descriptions and README (follow-up to #124) #137

Problem description
Wording leftovers and typos in the r2.2 snapshot:

• README.md L20-L21: "the customer's data usage status" (twice), not replaced as asked in Leftover copy-paste and role-terminology wording (device-data-volume.yaml, subscriptions.yaml, README) #124. device-data-volume.yaml L5 uses "the end user's data usage status".
• device-data-volume-subscriptions.yaml L17: "this choice indicated to the API consumer in the response along with the connected network type information"; this API returns no network type information.
• Typos:
  • "Date Allowance": device-data-volume.yaml L19, device-data-volume-subscriptions.yaml L19
  • "could be send": device-data-volume-subscriptions.yaml L37
  • "may used": device-data-volume-subscriptions.yaml L42

Expected action

• Replace "the customer's" in README.md with "the end user's", as in device-data-volume.yaml.
• Correct the other listed wording.

Additional context
Found during release review of #133. Follow-up to #124.