[gdb/cli] Fix help text of maint info target-sections

I noticed a typo:
...
(gdb) help maint info target-sections
List GDB's internal section table.

Print the current targets section list.  This is a sub-set of all
sections, from all objects currently loaded.  Usually the ALLOC
sectoins.
...

Fix this by using "sections".

Reviewed-By: Tom Tromey <tom@tromey.com>