[gdb/doc] Fix print inferior-events default

[gdb/doc] Fix print inferior-events default

In the docs about print inferior-events we read:
...
By default, these messages will not be printed.
...

That used to be the case, but is no longer so since commit f67c0c91715 "Enable
'set print inferior-events' and improve detach/fork/kill/exit messages".

Fix this by updating the docs.