Change manual build instructions to fit most common usage

[ROCm/rccl commit: 00a910c2da]

projects/rccl/README.md

@@ -37,10 +37,14 @@ $ git clone https://github.com/ROCmSoftwarePlatform/rccl.git
 $ cd rccl
 $ mkdir build
 $ cd build
-$ CXX=/opt/rocm/bin/hcc cmake -DCMAKE_INSTALL_PREFIX=$PWD/rccl-install ..
+$ CXX=/opt/rocm/bin/hcc cmake ..
 $ make -j 8
 ```
-You may substitute a path of your own choosing for CMAKE_INSTALL_PREFIX. Note: ensure rocm-cmake is installed, `apt install rocm-cmake`.
+You may substitute an installation path of your own choosing by passing CMAKE_INSTALL_PREFIX. For example:
+```shell
+$ CXX=/opt/rocm/bin/hcc cmake -DCMAKE_INSTALL_PREFIX=$PWD/rccl-install ..
+```
+Note: ensure rocm-cmake is installed, `apt install rocm-cmake`.
 
 #### To build the RCCL package and install package :