message: fixed typo in index
diff: 1. [Overview](README.md#1-overview)
  2. [Architecture](README.md#2-architecture)
  3. [Prerequisites](README.md#3-prerequisites)
  - 4. [Start Hyperledger Fabric network](README.md#5-start-hyperledger-fabric-network)
  + 4. [Start Hyperledger Fabric network](README.md#4-start-hyperledger-fabric-network)
  5. [Monitor Hyperledger Fabric network](README.md#5-monitor-hyperledger-fabric-network)
  6. [Troubleshooting](README.md#6-troubleshooting)
  