message: README.md: fix quic/ subdir listing
  Summary:
  Trivial update to README.md: Fix a typo (sample->sample) and put the subdirs in alphabetical order.
  Pull Request resolved:
diff: ## Source Layout
  - `quic/api`: Defines API that applications can use to interact with the QUIC transport layer.
  - `quic/client`: Client transport implementation
  -- `quic/server`: Server transport implementation
  - `quic/codec`: Read and write codec implementation for the protocol
  - `quic/common`: Implementation of common utility functions
  - `quic/congestion_control`: Implementation of different congestion control algorithms such as Cubic and Copa
  - `quic/flowcontrol`: Implementations of flow control functions
  - `quic/handshake`: Implementations cryptographic handshake layer
  -- `quic/loss`: Implementations of different loss recovery algorithms
  -- `quic/logging`: Implementation of logging framework
  - `quic/happyeyeballs`: Implementation of mechanism to race IPV4 and IPV6 connection and pick a winner
  +- `quic/logging`: Implementation of logging framework
  +- `quic/loss`: Implementations of different loss recovery algorithms
  +- `quic/samples`: Example client and server
  +- `quic/server`: Server transport implementation
  - `quic/state`: Defines and implements both connection and stream level state artifacts and state machines
  -- `quic/sample`: Example client and server
  ## Dependencies
  