[Qt] Clarify: smartfee button name "Minimize"

Make clear this button hides the options instead of minimizing the transaction fee.

src/qt/forms/sendcoinsdialog.ui

@@ -807,7 +807,7 @@
              <string>collapse fee-settings</string>
             </property>
             <property name="text">
-             <string>Minimize</string>
+             <string>Hide</string>
             </property>
            </widget>
           </item>