selinux: fix typo in filesystem name

Correct the filesystem name to "binder" to enable genfscon per-file
labelling for binderfs.

Fixes: 7a4b5194 ("selinux: allow per-file labelling for binderfs")
Signed-off-by: Hridya Valsaraju <hridya@google.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
[PM: slight style changes to the subj/description]
Signed-off-by: Paul Moore <paul@paul-moore.com>