add default value to default config

Author: jonastheis

rollup/conf/config.json

@@ -94,6 +94,7 @@
       "propose_interval_milliseconds": 100,
       "max_block_num_per_chunk": 100,
       "max_tx_num_per_chunk": 100,
+      "max_l2_gas_per_chunk": 20000000,
       "max_l1_commit_gas_per_chunk": 11234567,
       "max_l1_commit_calldata_size_per_chunk": 112345,
       "chunk_timeout_sec": 300,