docs: add an example binding for normalizing Unicode to the sample nanorc

This requires `uconv` from the 'icu-devtools' package (on Debian).

doc/sample.nanorc.in

@@ -295,6 +295,9 @@
 ## For copying a marked region to the system clipboard:
 # bind Sh-M-C "{execute}|xsel -ib{enter}{undo}" main
 
+## For normalizing Unicode to precomposed characters:
+# bind Sh-M-N "{execute}| uconv -x nfc {enter}" main
+
 ## For snipping trailing blanks when you save a file:
 # bind ^S "{execute}| sed 's/\s\+$//' {enter}{savefile}" main