Fix typo on documentation ("show set startup-with-shell")

Fix typo on documentation ("show set startup-with-shell")

The documentation was erroneously saying that there is a command named
"show set startup-with-shell", while the correct version is "show
startup-with-shell".  This commit fixes obvious mistake.

gdb/doc/ChangeLog:
2017-08-12  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/21925
	* gdb.texinfo (Starting) <startup-with-shell>: Fix typo ("show
	set...").