Path to building instructions in README.md hotfix

Change-Id: I79829609407eaebefbccadb3e59adfffeab2265a

Author: pszymich

README.md

@@ -32,7 +32,7 @@ IGC is supported on the following 64 bit Linux operating systems:
 
 ## Building
 
-* [Ubuntu](https://github.com/intel/intel-graphics-compiler/master/documentation/build_ubuntu.md)
+* [Ubuntu](https://github.com/intel/intel-graphics-compiler/blob/master/documentation/build_ubuntu.md)
 
 ## Supported Platforms