Fix prompt build

Author: frank-at-adacore

courses/ada_essentials/135_visibility/lab/visibility/prompt/default.gpr

@@ -1,5 +1,5 @@
 project Default is
-   for Main use ("main1.adb", "main2.adb");
+   for Main use ("main.adb");
    for Source_Dirs use (".");
    package Compiler is
      for Switches ("ada") use ("-g","-gnata","-gnatyr");