[vte] app: Improve option value description

[Christian Persch <chpe src gnome org>]

commit 51cd95caa2cf63270c51d6df027afe0252487ccd
Author: Christian Persch <chpe src gnome org>
Date:   Sat Oct 23 20:55:15 2021 +0200

    app: Improve option value description

 src/app/app.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/app/app.cc b/src/app/app.cc
index 1767f919..b0c3f3d2 100644
--- a/src/app/app.cc
+++ b/src/app/app.cc
@@ -590,7 +590,7 @@ public:
                         { "extra-margin", 0, 0, G_OPTION_ARG_INT, &extra_margin,
                           "Add extra margin around the terminal widget", "MARGIN" },
                         { "fd", 0, 0, G_OPTION_ARG_CALLBACK, (void*)parse_fd,
-                          "Pass file descriptor N (as M) to the child process", "N[=M]" },
+                          "Pass file descriptor N (as M) to the child process", "N[:M]|N[=]M]" },
                         { "feed-stdin", 'B', 0, G_OPTION_ARG_NONE, &feed_stdin,
                           "Feed input to the terminal", nullptr },
                         { "font", 'f', 0, G_OPTION_ARG_STRING, &font_string,