docs: suggest a setting for 'stripecolor' in the sample nanorc

doc/sample.nanorc.in

@@ -202,6 +202,7 @@
 # set statuscolor brightwhite,green
 # set errorcolor brightwhite,red
 # set selectedcolor brightwhite,magenta
+# set stripecolor ,yellow
 # set numbercolor cyan
 # set keycolor cyan
 # set functioncolor green
@@ -210,6 +211,7 @@
 # set statuscolor brightwhite,magenta
 # set errorcolor brightwhite,red
 # set selectedcolor brightwhite,cyan
+# set stripecolor ,yellow
 # set numbercolor magenta
 # set keycolor brightmagenta
 # set functioncolor magenta