Update the path to clean_header.py on the documentation.

#     etc.

#

# NOTE: These directories MUST contain post-processed headers using the

# bionic/libc/kernel/clean_header.py tool. Additionally, the original kernel

# headers must also be checked in, but in a different subdirectory. By

# bionic/libc/kernel/tools/clean_header.py tool. Additionally, the original

# kernel headers must also be checked in, but in a different subdirectory. By

# convention, the originals should be checked into original-kernel-headers

# directory of the same parent dir. For example,

#     device/samsung/tuna/kernel-headers            <----- post-processed