fix nbconvert completion

thanks @jakobgager for pointing this out in  #4528

examples/core/ipython-completion.bash

@@ -46,7 +46,7 b' _ipython()'
 
     if [[ ${cur} == -* ]]; then
         case $mode in
-            "notebook" | "qtconsole" | "console" | "kernel")
+            "notebook" | "qtconsole" | "console" | "kernel" | "nbconvert")
                 _ipython_get_flags $mode
                 opts=$"${opts} ${baseopts}"
                 ;;