ref-manual: Fixed SRC_URI cut-and-paste error.

The update fixes a copy-paste-and-forget-to-update-everything error
in the documentation of the `SRC_URI` entry in the Reference Manual.
The `maxdate` paragraph just repeated what is there for the
neighboring `mindate` paragraph which was incorrect.

(From yocto-docs rev: 26c5549580e108c40aa7c95860d12c675596820c)

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

documentation/ref-manual/ref-variables.xml

@@ -13301,7 +13301,7 @@ recipes-graphics/xorg-font/font-alias_1.0.3.bb:PR = "${INC_PR}.3"
                             </para></listitem>
                         <listitem><para><emphasis><filename>maxdate</filename> -</emphasis>
                             Apply the patch only if <filename>SRCDATE</filename>
-                            is not later than <filename>mindate</filename>.
+                            is not later than <filename>maxdate</filename>.
                             </para></listitem>
                         <listitem><para><emphasis><filename>minrev</filename> -</emphasis>
                             Apply the patch only if <filename>SRCREV</filename>