need to clarify `SetCustomMiningJob.Success` description

[plebhash]

while reviewing stratum-mining/stratum#1243 I noticed we missed a tiny detail on #98

current wording for the description of SetCustomMiningJob.Success is a bit misleading

Response from the server when it accepts the custom mining job. Client can start to mine on the job immediately (by using the job_id provided within this response).

it should be:

Response from the server when it accepts the custom mining job. Client can start submitting shares for this job immediately (by using the job_id provided within this response).