[FEM] fix button text

- looking at the Git log this button never had a plot functionality
- for the discussion see: https://forum.freecadweb.org/viewtopic.php?f=18&t=69189

src/Mod/Fem/Gui/Resources/ui/ResultShow.ui

@@ -328,7 +328,7 @@
       <item>
        <widget class="QPushButton" name="calculate">
         <property name="text">
-         <string>Calculate and plot</string>
+         <string>Calculate</string>
         </property>
        </widget>
       </item>