[fuzzing] fix README and add instructions for reproducing bugs
[fuzzing] fix README and add instructions for reproducing bugs

- Fixing the README to build single fuzzer (we changed the env var required for this step)
- add one more step to reproducing bugs and link to google-oss fuzz doc

Closes: #3044
Approved by: sausagee

aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core