fixed link to wiki

README.md

 # Marlin 3D Printer Firmware
 <img align="top" width=175 src="Documentation/Logo/Marlin%20Logo%20GitHub.png" />
- Additional documentation can be found in [The Marlin Wiki](https://github.com/MarlinFirmware/Marlin/wiki/Main-Page).
+ Additional documentation can be found in [The Marlin Wiki](https://github.com/MarlinFirmware/Marlin/wiki).
 
 ## Release Candidate -- Marlin 1.1.0-RC5 - 01 April 2016