Nit: Update Spiedermonkey documentation (#475)

Author: TobiasWienand

Targets/Spidermonkey/README.md

@@ -3,5 +3,5 @@
 To build Spidermonkey for fuzzing:
 
 1. Clone the Firefox mirror from https://github.com/mozilla/gecko-dev
-2. Run the fuzzbuild.sh script in the js/src directory of the firefox checkout
-3. fuzzbuild_OPT.OBJ/dist/bin/js will be the JavaScript shell for the fuzzer
+2. Run the fuzzbuild.sh script in the gecko-dev root directory of the firefox checkout
+3. gecko-dev/obj-fuzzbuild/dist/bin/js will be the JavaScript shell for the fuzzer