proc.5: Update reference to kernel's crypto API documentation

proc.5: Update reference to kernel's crypto API documentation

DocBook documentation has been removed in commit
v4.13-rc1~34^2~21^2~11.  Crypto API has been converted to
ReStructured format during the Linux 4.10 development cycle
(see commits v4.10-rc1~40^2~8 and v4.10-rc1~40^2~7).

* man5/proc.5 (.SS Files and directories) <.TP .I /proc/crypto>:
  Amend the reference to the kernel's crypto API documentation
  with the new location, effective since Linux 4.10.

Signed-off-by: Eugene Syromyatnikov <evgsyr@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>