Remove incorrect env variables from README example

[pistomat]

Fixes #56

Removing the environment variables works, the alternative is setting them to correct values. The L1 private key correct value is the default one:

export L1_PRIVKEY=6c45335a22461ccdb978b78ab61b238bad2fae4544fb55c14eb096c875ccfc52

But I don't know what the BUILDER_URL should be. Setting it to http://host.docker.internal:1234 does not work.

[dmarzzz]

@pistomat I believe the private key should be set to the key that the genesis file pre-allocates eth to cc @ferranbt