Correct menu Insert>Trend line

Only 1 trend line can be inserted. It is no more possible
to insert trend lines for all series at a time.
See comments in b0fbaa6f

Change-Id: I3a8548af3d3cd641b4da342ee03681647844fde3
Reviewed-on: https://gerrit.libreoffice.org/28226Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>

officecfg/registry/data/org/openoffice/Office/UI/ChartCommands.xcu

@@ -37,7 +37,7 @@
       </node>
       <node oor:name=".uno:InsertMenuTrendlines" oor:op="replace">
         <prop oor:name="Label" oor:type="xs:string">
-          <value xml:lang="en-US">Tre~nd Lines...</value>
+          <value xml:lang="en-US">Tre~nd Line...</value>
         </prop>
       </node>
       <node oor:name=".uno:InsertMenuMeanValues" oor:op="replace">