catch exceptions as const

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ib9f55b5a26f9a48838759cfaef73c44181242ac6
diff --git a/gpioMonMain.cpp b/gpioMonMain.cpp
index 4b8b052..f8084a3 100644
--- a/gpioMonMain.cpp
+++ b/gpioMonMain.cpp
@@ -59,7 +59,7 @@
     {
         app.parse(argc, argv);
     }
-    catch (CLI::Error& e)
+    catch (const CLI::Error& e)
     {
         return app.exit(e);
     }