[gdb/doc] Fix "maint info selftests" command index entry

[gdb/doc] Fix "maint info selftests" command index entry

Currently, the entry for the command "maint info selftests" in the Command,
Variable, and Function Index is listed at '"', rather than next to the other
"maint info" commands.

Fix this by removing the superfluous quoting in the @kindex entry.

gdb/doc/ChangeLog:

2019-05-14  Tom de Vries  <tdevries@suse.de>

	* gdb.texinfo (Maintenance Commands): Remove superfluous
	quoting on command index entry for "maint info selftests".