explicitly use ATHENA_PREFIX

src/docs/part2/adding_detectors.md

@@ -16,7 +16,7 @@ cd part2
 ### Compile the detector library
 
 ```bash
-cmake -B build -S . -DCMAKE_INSTALL_PREFIX=../local/ -DCMAKE_CXX_STANDARD=17
+cmake -B build -S . -DCMAKE_INSTALL_PREFIX=$ATHENA_PREFIX -DCMAKE_CXX_STANDARD=17
 cmake --build build -j4 -- install
 ```