docs/Xcode: there is no 10.11 CLT 8.0

From the 8.1 Xcode Beta:
```
There is no Command Line Tools (OS X 10.11) for Xcode 8 package. Xcode 8 contains
SDKs that are incompatible with earlier toolchains. Developers who want to make
use of the Xcode 8 SDKs from the command line must choose the SDK with `xcode-select`.
Developers on OS X El Capitan who have installed versions of the Command Line Tools
(OS X 10.11) for Xcode 8 Beta should install Command Line Tools (OS X 10.11)
for Xcode 7.3.1.
```

Thanks Apple.

docs/Xcode.md

@@ -10,7 +10,7 @@ Tools available for your platform:
  10.8  | 5.1.1 | April 2014
  10.9  | 6.2   | 6.2
  10.10 | 7.2.1 | 7.2
- 10.11 | 8.0   | 8.0
+ 10.11 | 8.0   | 7.3
  10.12 | 8.0   | 8.0