Update README.md (still referring to gcc-8)

Author: KrisThielemans

README.md

@@ -257,7 +257,7 @@ You can enable more features by using other CMake variables (e.g. `ROOT_X11` def
 
 ## Use a different compiler than the system default
 
-You can tell CMake to use a different compiler than what it finds by default. You will need to specify both C and C++ compilers. For instance, to use `gcc8`, use
+You can tell CMake to use a different compiler than what it finds by default. You will need to specify both C and C++ compilers. For instance, to use `gcc-11`, use
 
 ```sh
 CC=gcc-11