message: fixed small typo in variant docs example code
diff: @@ -143,12 +143,12 @@ Given the following variants file:
  vulkan:
  short: Vulkan
  long: Vulkan rendering
  - setting:
  + settings:
  ENGINE: Vulkan
  software:
  short: Software
  long: Software rendering
  - setting:
  + settings:
  ENGINE: Software
  network:
  @@ -157,17 +157,17 @@ Given the following variants file:
  boost:
  short: Boost.Asio
  long: Use Boost.Asio for networking
  - setting:
  + settings:
  NETWORK: Boost
  asio:
  short: Asio
  long: Use standalone-Asio for networking
  - setting:
  + settings:
  NETWORK: Asio
  net-ts:
  short: NetTS
  long: Use the C++ Networking TS for networking
  - setting:
  + settings:
  NETWORK: net-ts
  ```
  