docs(first): fix a typo in first.ts
docs(first): fix a typo in first.ts

SangKa/RxJS-Docs-CN,SangKa/RxJS-Docs-CN,SangKa/RxJS-Docs-CN,RxJS-CN/RxJS-Docs-CN,RxJS-CN/RxJS-Docs-CN,SangKa/RxJS-Docs-CN,RxJS-CN/RxJS-Docs-CN,RxJS-CN/RxJS-Docs-CN