doc: Fix fuzz test_runner.py path

This commit fixes the path listed in the documentation for the fuzz
testing test_runner.py. Previously the --help option worked but running
fuzz tests from the documented path did not.

Author: monlovesmango

doc/fuzzing.md

@@ -19,7 +19,7 @@ One can use `--preset=libfuzzer-nosan` to do the same without common sanitizers
 See [further](#run-without-sanitizers-for-increased-throughput) for more information.
 
 There is also a runner script to execute all fuzz targets. Refer to
-`./test/fuzz/test_runner.py --help` for more details.
+`./build_fuzz/test/fuzz/test_runner.py --help` for more details.
 
 ## Overview of Bitcoin Core fuzzing