manual: add _TARGET_FINALIZE_HOOKS in the list of hooks

While reading the docs to find hooks, I completely missed the
LIBFOO_TARGET_FINALIZE_HOOKS one which was actually matching my
use-case.

Though it is documented in a subsection a few lines below, let's also
have it in the list of supported hooks so it's not hidden away.

Cc: Quentin Schulz <foss+buildroot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

docs/manual/adding-packages-hooks.txt

@@ -46,6 +46,8 @@ The following hook points are available:
 * +LIBFOO_PRE_LEGAL_INFO_HOOKS+
 * +LIBFOO_POST_LEGAL_INFO_HOOKS+
 
+* +LIBFOO_TARGET_FINALIZE_HOOKS+
+
 These variables are 'lists' of variable names containing actions to be
 performed at this hook point. This allows several hooks to be
 registered at a given hook point. Here is an example: