CLI: Fix declared return type of pbjs/pbts callback (#1025)

Author: vuryleo

Diff for: cli/pbjs.d.ts

@@ -1,4 +1,4 @@
-type pbjsCallback = (err: Error|null, output?: string) => {};
+type pbjsCallback = (err: Error|null, output?: string) => void;
 
 /**
  * Runs pbjs programmatically.

Diff for: cli/pbts.d.ts

@@ -1,4 +1,4 @@
-type pbtsCallback = (err: Error|null, output?: string) => {};
+type pbtsCallback = (err: Error|null, output?: string) => void;
 
 /**
  * Runs pbts programmatically.