That's not something configurable in the GRUB configuration files. You would need to recompile GRUB2. This string is defined in normal/main.c (the msg variable; see line 539 in the Ubuntu version; the Fedora version should be near but may not be on exactly the same line). Altering it might be as simple as changing that line and recompiling.
I highly recommend creating your own RPM package(s) with your modified source. That way you can still use your package manager to handle updates. See for example Packaging software with RPM.