docs(single): fix a typo
docs(single): fix a typo

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