fix(gui): use singular form in OpenAction warning dialog to match MIN_INOUT_ELEMENTS=1

Addresses review comment PRRT_kwDOQyJ9Cc50Kko4 — dialog text in OpenAction
used "element(s)" which doesn't match the constant (always 1). Aligned with
the wording already used in performSave() dialog.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: bedaHovorka

desktop-ui/src/main/kotlin/cz/vutbr/fit/interlockSim/gui/MenuBar.kt

@@ -109,7 +109,7 @@ class MenuBar : JMenuBar() {
 									"WARNING: This railway network has insufficient InOut elements " +
 										"($inOutsCount found, $minRequired required).\n\n" +
 										"The editor will prevent saving this context until you add at least " +
-										"$minRequired InOut element(s).\n\n" +
+										"$minRequired InOut element (entry/exit point).\n\n" +
 										"InOut elements define entry/exit points for trains.",
 									"Validation Warning",
 									JOptionPane.WARNING_MESSAGE