Fixed a few grammar issues in README
Fixed a few grammar issues in README

Copy of PR from master branch, fixed a few grammar issues in README to increase readability.
shiratsu/RxSwift,ReactiveX/RxSwift,cfraz89/RxSwift,cfraz89/RxSwift,shiratsu/RxSwift,tottakai/RxSwift,KyoheiG3/RxSwift,KyoheiG3/RxSwift,ReactiveX/RxSwift,shiratsu/RxSwift,kzaher/RxSwift,kzaher/RxSwift,kzaher/RxSwift,cfraz89/RxSwift,tottakai/RxSwift,tottakai/RxSwift,KyoheiG3/RxSwift,kzaher/RxSwift,ReactiveX/RxSwift,cfraz89/RxSwift