docs(misc): fix typo, add prep-iroha help message

CONTRIBUTING.md

@@ -44,7 +44,7 @@ deno task prep:iroha --path /path/to/local/iroha/clone
 
 ```sh
 # clone the repo
-deno task prep:iroha --git https://github.com/hyperledger-iroha/iroha.git --rev v2.0.0-rc.1.0
+deno task prep:iroha --git https://github.com/hyperledger-iroha/iroha.git --git-rev v2.0.0-rc.1.0
 ```
 
 After Iroha is linked, you need to prepare some artifacts from it (binaries such as `irohad`, `kagami`, `iroha_codec`;

etc/task-prep-iroha.ts

@@ -131,6 +131,27 @@ await match(args)
     await copyArtifacts()
   })
   .otherwise(() => {
-    $.logError('Bad CLI args')
+    const cmd = colors.gray(`deno task prep:iroha`)
+
+    console.info(`Usage:
+
+  ${colors.blue('## Link Iroha')}
+
+  ${colors.magenta(`${cmd} --git <git repo> --git-rev <tag or revision>`)}
+    to clone Iroha repo
+
+  or
+
+  ${colors.magenta(`${cmd} --path <local path>`)}
+    to symlink a local Iroha repo
+
+  ${colors.blue('## Build')}
+
+  ${colors.magenta(`${cmd} --build`)}
+    to build Iroha artifacts
+
+  ${colors.magenta(`${cmd} --check`)}
+    to check that the artifacts are ready
+  `)
     Deno.exit(1)
   })