Fixed wrong toolchain file path in js_build.sh (Chia-Network#240)

Author: ChiaMineJP

Diff for: js_build.sh

@@ -5,5 +5,5 @@ git submodule update --init --recursive
 mkdir js_build
 cd js_build
 
-cmake ../ -DCMAKE_TOOLCHAIN_FILE=$(realpath $(which emcc))/cmake/Modules/Platform/Emscripten.cmake
+cmake ../ -DCMAKE_TOOLCHAIN_FILE=$(dirname $(realpath $(which emcc)))/cmake/Modules/Platform/Emscripten.cmake
 cmake --build . --