Instructions missing one step that doesn't create ./bench

Suggest adding a missing line in the install documentation marked [missing] as shown below to avoid, ./bench not found.

$ env RUSTFLAGS="-C target-cpu=native -g" FFI_BUILD_FROM_SOURCE=1 make clean deps bench
go build -o bench ./cmd/lotus-bench **[missing]**
$ env FIL_PROOFS_MAXIMIZE_CACHING=1 RUST_LOG=info TMPDIR=/storage ./bench sealing --storage-dir /storage/bench --sector-size 32GiB 2>&1 | tee bench.log