linuxManualConfig: fix inaccurate FIXME comment

I read this comment, checked out 4.19, saw that it didn't have any
instances of /bin/pwd, and rejoiced, assuming that the removal must
have been backported to the affected kernels.  But actually, /bin/pwd
was just removed earlier, in 4.15, so kernels older than that are
still affected.

Fix the version number in the comment so I don't go through exactly
the same process again a few months from now next time I'm working on
this file.

pkgs/os-specific/linux/kernel/manual-config.nix

@@ -119,7 +119,7 @@ let
       postPatch = ''
         sed -i Makefile -e 's|= depmod|= ${buildPackages.kmod}/bin/depmod|'
 
-        # fixup for pre-5.4 kernels using the $(cd $foo && /bin/pwd) pattern
+        # fixup for pre-4.15 kernels using the $(cd $foo && /bin/pwd) pattern
         # FIXME: remove when no longer needed
         substituteInPlace Makefile tools/scripts/Makefile.include --replace /bin/pwd pwd