training_timer: Use regular colon as time divider

Use „:” (U+003A) instead of „∶” (U+2236). This will fix the reversed
time in RTL languages.

See
https://gitlab.gnome.org/Teams/Websites/developer.gnome.org-hig/-/issues/127
for more information.

src/training_timer.rs

@@ -118,7 +118,7 @@ fn remaining_str_colon(remaining_s: usize) -> String {
         // Translators: Shown in the timer page when the training has come to the end
         gettext("Finished!")
     } else {
-        String::from("∶")
+        String::from(":")
     }
 }