Update config.example to state "librem" as possible model

When we enabled support for Librem Key device we forgot to update the
configuration file example to state the "librem" model variant. This
change updates the corresponding comment in there.

doc/config.example.toml

@@ -2,7 +2,8 @@
 # ${XDG_CONFIG_HOME}/nitrocli/config.toml, where XDG_CONFIG_HOME defaults to
 # ${HOME}.
 
-# The model to connect to (string, "pro" or "storage", default: not set).
+# The model to connect to (string, "pro", "storage", or "librem", default:
+# not set).
 model = "pro"
 # The serial number of the device to connect to (list of strings, default:
 # empty).