Update README.md (#95)

Master branch is no more, rename to main

Author: 0xTim

README.md

@@ -77,7 +77,7 @@ Check out [CONTRIBUTORS.txt](https://github.com/swift-server/RediStack/blob/main
 
 | Proposal | Pitch | Discussion | Review | Vote |
 |:---:|:---:|:---:|:---:|:---:|
-| [SSWG-0004](https://github.com/swift-server/sswg/blob/master/proposals/0004-nio-redis.md) | [2019-01-07](https://forums.swift.org/t/swiftnio-redis-client/19325) | [2019-04-01](https://forums.swift.org/t/discussion-nioredis-nio-based-redis-driver/22455) | [2019-06-09](https://forums.swift.org/t/feedback-redisnio-a-nio-based-redis-driver/25521) | [2019-06-27](https://forums.swift.org/t/june-27th-2019/26580) |
+| [SSWG-0004](https://github.com/swift-server/sswg/blob/main/proposals/0004-nio-redis.md) | [2019-01-07](https://forums.swift.org/t/swiftnio-redis-client/19325) | [2019-04-01](https://forums.swift.org/t/discussion-nioredis-nio-based-redis-driver/22455) | [2019-06-09](https://forums.swift.org/t/feedback-redisnio-a-nio-based-redis-driver/25521) | [2019-06-27](https://forums.swift.org/t/june-27th-2019/26580) |
 
 ## Language and Platform Support