Fix broken link in README

Signed-off-by: Santiago Castro <santi.1410@hotmail.com>

README.md

@@ -83,7 +83,7 @@ Requirements:
 
 -   Go 1.6 or higher
 -   A [working golang](https://golang.org/doc/code.html) environment
--   [Protobuf 3.x or higher] (https://developers.google.com/protocol-buffers/docs/downloads) to regenerate protocol buffer files (e.g. using `make generate`)
+-   [Protobuf 3.x or higher](https://developers.google.com/protocol-buffers/docs/downloads) to regenerate protocol buffer files (e.g. using `make generate`)
 
 *SwarmKit* is built in Go and leverages a standard project structure to work well with Go tooling.
 If you are new to Go, please see [BUILDING.md](BUILDING.md) for a more detailed guide.