Fix export genesis command (#395)

cumulus/rococo-parachains/src/cli.rs

@@ -60,8 +60,10 @@ pub struct ExportGenesisStateCommand {
 	pub output: Option<PathBuf>,
 
 	/// Id of the parachain this state is for.
-	#[structopt(long, default_value = "100", conflicts_with = "chain")]
-	pub parachain_id: u32,
+	///
+	/// Default: 100
+	#[structopt(long, conflicts_with = "chain")]
+	pub parachain_id: Option<u32>,
 
 	/// Write output in binary. Default is to write in hex.
 	#[structopt(short, long)]

cumulus/rococo-parachains/src/command.rs

@@ -241,7 +241,7 @@ pub fn run() -> Result<()> {
 
 			let block: Block = generate_genesis_block(&load_spec(
 				&params.chain.clone().unwrap_or_default(),
-				params.parachain_id.into(),
+				params.parachain_id.unwrap_or(100).into(),
 			)?)?;
 			let raw_header = block.header().encode();
 			let output_buf = if params.raw {