fix: incomplete command in fuzzing README.md

Author: cktii

fuzzing/README.md

@@ -1,12 +1,14 @@
 # Fuzzing BehaviorTree.CPP
 
-You can build the existing harnesses either for libfuzzer or AFL++:
+You can build the existing harnesses either for libfuzzer or AFL++.
+Building the fuzzers requires `clang` (libfuzzer) or an installed version
+of [AFL++](https://github.com/AFLplusplus/AFLplusplus).
 
 ## libfuzzer
 
 ```bash
 mkdir build_libfuzzer && cd build_libfuzzer
-cmake -DENABLE_FUZZING ..
+cmake -DENABLE_FUZZING=ON ..
 ```
 
 ## AFL++
@@ -15,5 +17,5 @@ cmake -DENABLE_FUZZING ..
 export CC=afl-clang-fast
 export CXX=afl-clang-fast++
 mkdir build_afl && cd build_afl
-cmake -DENABLE_FUZZING -DUSE_AFLPLUSPLUS ..
+cmake -DENABLE_FUZZING=ON -DUSE_AFLPLUSPLUS=ON ..
 ```