Fix for "config(8) "kernel building" instructions lead to bad kernel name (which breaks relink)"

[IL Ka]

See:
https://marc.info/?l=openbsd-bugs&m=152380023916176&w=2


diff --git usr.sbin/config/config.8 usr.sbin/config/config.8
index 748aaab7a..10fdf7b92 100644
--- usr.sbin/config/config.8
+++ usr.sbin/config/config.8
@@ -341,6 +341,7 @@ mounted on
 do the following:
 .Bd -literal -offset indent
 # cd /somedir
+# mkdir SOMEFILE && cd SOMEFILE
 # cp /usr/src/sys/arch/somearch/conf/SOMEFILE .
 # vi SOMEFILE  (to make any changes)
 # config -s /usr/src/sys -b . SOMEFILE