rl-1509.xml: Gnome -> GNOME

Changed files
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nixos
doc
manual
release-notes
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nixos/doc/manual/release-notes/rl-1509.xml
···
</listitem>
<listitem>
-
<para>Gnome has been upgraded to 3.16.
+
<para>GNOME has been upgraded to 3.16.
</para>
</listitem>