doc/submitting-changes.xml: suggest "nixos/<module>" prefix for NixOS changes

Commit 8537cf0f818a95073e93ac755e9fa17c30982caa
("CONTRIBUTING.md: suggest "nixos/<module>" prefix for NixOS changes")
only changed CONTRIBUTING.md file and forgot about the Nixpkgs manual.
(I didn't know this information was stored in two places.)

Changed files
+4 -4
doc
+4 -4
doc/submitting-changes.xml
···
<listitem>
<para>Format the commit in a following way:</para>
<programlisting>
-
(pkg-name | service-name): (from -> to | init at version | refactor | etc)
+
(pkg-name | nixos/&lt;module>): (from -> to | init at version | refactor | etc)
Additional information.
</programlisting>
···
<listitem>
<para>
-
<command>hydra service: add bazBaz option</command>
+
<command>nixos/hydra: add bazBaz option</command>
</para>
</listitem>
<listitem>
<para>
-
<command>nginx service: refactor config generation</command>
+
<command>nixos/nginx: refactor config generation</command>
</para>
</listitem>
</itemizedlist>
···
<itemizedlist>
<listitem>
-
<para>Write the title in format <command>(pkg-name | service): improvement</command>.
+
<para>Write the title in format <command>(pkg-name | nixos/&lt;module>): improvement</command>.
<itemizedlist>
<listitem>