docs: Update txaccepted method in json_rpc_api.md.
docs: Update txaccepted method in json_rpc_api.md.

The 'txaccepted' method is currently showing an out of date parameter within json_rpc_api.md. The first parameter is currently set to 'TxSha', however it should be set to either 'TxID' or 'TxHash' .
jrick/dcrd,decred/dcrd,jrick/dcrd,decred/dcrd