ref-manual: Removed non-existant variable from PACKAGE_WRITE_DEPS description

Took out an occurrence of PACKAGE_WRITE_DEPENDS

(From yocto-docs rev: f7b2c7c08f3f504e14943a01e7fc0a73c96e9636)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-variables.xml

@@ -9998,7 +9998,7 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
                     execute at rootfs creation time rather than on the
                     target but depends on a native tool in order to execute,
                     you need to list the tools in
-                    <filename>PACKAGE_WRITE_DEPENDS</filename>.
+                    <filename>PACKAGE_WRITE_DEPS</filename>.
                 </para>
 
                 <para>