make acornOptions optional in parse() in docs (#3237)

because [in the types it is `any`](https://github.com/rollup/rollup/blob/5289fdedf878b301b3337866d0892e207874c7ce/src/rollup/types.d.ts#L178) and in the source [it is optional](https://github.com/rollup/rollup/blob/5289fdedf878b301b3337866d0892e207874c7ce/src/Graph.ts#L67)

docs/05-plugin-development.md

@@ -446,7 +446,7 @@ for (const moduleId of this.moduleIds) { /* ... */ }
 
 or converted into an Array via `Array.from(this.moduleIds)`.
 
-#### `this.parse(code: string, acornOptions: AcornOptions) => ESTree.Program`
+#### `this.parse(code: string, acornOptions?: AcornOptions) => ESTree.Program`
 
 Use Rollup's internal acorn instance to parse code to an AST.