Doco patch: fixes ada/9406

[Ben Elliston]

>>>>> "Geert" == Geert Bosch <bosch@gnat.com> writes:

  >> A simple doco patch, but I wanted to run the terminology change past
  >> some Ada maintainers.  Okay to commit to mainline?

  Geert> This patch does not seem an improvement at all.  Note that
  Geert> "main program" is used in the Ada 95 language standard, as
  Geert> well as the GNAT documentation, error messages etc.

  Geert> Furthermore, "entry unit" is a made up term that is confusing
  Geert> (an "entry" is a specific construct in Ada), and inconsistent
  Geert> with the language standard and documentation.

This sounds reasonable.  I will paste your comments into the PR and
close it.  Thanks,

Ben