gdb/doc: update examples in gdb.Progspace and gdb.Objfile docs

This commit updates the Python example code in the gdb.Progspace and
gdb.Objfile sections of the docs.  Changes made:

  1. Use @value{GDBP} for the GDB prompt rather than
  hard-coding (gdB),

  2. Use @group...@end group to split the example code into
  unbreakable chunks, and

  3. Add parenthesis to the Python print() calls in the examples.  In
  Python 2 it was OK to drop the parenthesis, but now GDB is Python 3
  only, example code should include the parenthesis.

Approved-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>