doc: fix quick-start executor command

[westhide]

Which issue does this PR close?

Closes N/A.

fix quick-start executor command

Start Ballista Executor fail because bind-grpc-port already in use

RUST_LOG=info ./target/release/ballista-scheduler

RUST_LOG=info ./target/release/ballista-executor -c 2 -p 50051

RUST_LOG=info ./target/release/ballista-executor -c 2 -p 50052

Executor

ballista_executor::executor_process: "executor services stopped with reason Err(TonicError(tonic::transport::Error(Transport, Os { code: 98, kind: AddrInUse, message: \"Address already in use\" })))"

Scheduler

ballista_scheduler::scheduler_server::grpc: Received executor stopped request from Executor 635ab9c4-ca05-4624-a5c9-c25893dd97c7 with reason 'executor services stopped with reason Err(TonicError(tonic::transport::Error(Transport, Os { code: 98, kind: AddrInUse, message: "Address already in use" })))'

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[milenkovicm]

Thanks @westhide