Modify lib.rs when deploying the Increment contract

[yskwmt]

Following this tutorial https://developers.stellar.org/docs/build/smart-contracts/getting-started/deploy-increment-contract, I encountered an error when I run this command ('id' and 'source' were appropriately replaced).

stellar contract invoke \
  --id CACDYF3CYMJEJTIVFESQYZTN67GO2R5D5IUABTCUG3HXQSRXCSOROBAN \
  --source alice \
  --network testnet \
  -- \
  increment

So I modified the lib.rs file created in the contracts/increment/src directory as follows.

[carstenjacobsen]

Closed with #1295