gdb/breakpoint: fix typo in help message of "set breakpoint condition-evaluation"

gdb/breakpoint: fix typo in help message of "set breakpoint condition-evaluation"

The options for the "breakpoint condition-evaluation" setting are
"host", "target", and "auto".  The help message mentions the option
"gdb" at one point instead of "host".  Fix this typo.  Also add a period.

gdb/ChangeLog:
2020-09-11  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* breakpoint.c: Fix typo in the help message of the
	"set breakpoint condition-evaluation" command.