[w3c/webauthn] 2c97d4: "with string-valued keys" => "whose keys are strin...

  Branch: refs/heads/issue-870-client-data-keys
  Home:   https://github.com/w3c/webauthn
  Commit: 2c97d4cf8f7a7f59472d9e76570f8608941f14b0
      https://github.com/w3c/webauthn/commit/2c97d4cf8f7a7f59472d9e76570f8608941f14b0
  Author: Emil Lundberg <emil@yubico.com>
  Date:   2018-04-24 (Tue, 24 Apr 2018)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  "with string-valued keys" => "whose keys are strings"

This helps avoid confusion by reading "string-valued keys" as "keys
mapped to values of string type".