Fix the name and version of macOS SDK

Author: takasek

Docs/GettingStarted.md

@@ -7,7 +7,7 @@ Although OS X is not a deployment platform for Swift Foundation, it is useful fo
 In order to build on OS X, you will need:
 
 * The latest version of Xcode
-* The latest version of the OS X SDK (at this time: 10.11)
+* The latest version of the macOS SDK (at this time: 10.13.2)
 * The [current Swift toolchain](https://swift.org/download/#snapshots).
 
 Foundation is developed at the same time as the rest of Swift, so the most recent version of the compiler is required in order to build it.