nnn: update page (#13605)

Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>
Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

pages/common/nnn.md

@@ -22,3 +22,7 @@
 - Sort files on [a]pparent disk usage / [d]isk usage / [e]xtension / [r]everse / [s]ize / [t]ime / [v]ersion:
 
 `nnn -T {{a|d|e|r|s|t|v}}`
+
+- Open a file you have selected. Select the file then press `o`, and type a program to open the file in:
+
+`nnn -o`