Update BUILDING.md (#2385)

I use PyEnv and rebuilding after recent commits was giving me a cmake error about not being able to find the Python3 executable. Advising other users who might run into this problem on how to fix it.

BUILDING.md

@@ -93,7 +93,7 @@ git submodule update --init
 # Copy the baserom to the OTRExporter folder
 cp <path to your ROM> OTRExporter
 # Generate Ninja project
-cmake -H. -Bbuild-cmake -GNinja # -DCMAKE_BUILD_TYPE:STRING=Release (if you're packaging)
+cmake -H. -Bbuild-cmake -GNinja # -DCMAKE_BUILD_TYPE:STRING=Release (if you're packaging) -DPython3_EXECUTABLE=$(which python3) (if you are using non-standard Python installations such as PyEnv)
 # Extract assets & generate OTR (run this anytime you need to regenerate OTR)
 cmake --build build-cmake --target ExtractAssets
 # Compile the project