Point to callback_future in callback docs

[Shadlock0133]

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Size Comparison

examples	master (KB)	pull request (KB)	diff (KB)	diff (%)
boids	172.771	172.771	0	0.000%
contexts	109.636	109.636	0	0.000%
counter	86.653	86.653	0	0.000%
counter_functional	87.303	87.303	0	0.000%
dyn_create_destroy_apps	89.769	89.769	0	0.000%
file_upload	102.620	102.620	0	0.000%
function_memory_game	167.344	167.344	0	0.000%
function_router	350.562	350.562	0	0.000%
function_todomvc	161.993	161.993	0	0.000%
futures	226.573	226.573	0	0.000%
game_of_life	107.511	107.511	0	0.000%
inner_html	83.687	83.687	0	0.000%
js_callback	112.874	112.874	0	0.000%
keyed_list	195.036	195.036	0	0.000%
mount_point	86.281	86.281	0	0.000%
nested_list	115.913	115.913	0	0.000%
node_refs	90.452	90.452	0	0.000%
password_strength	1539.073	1539.073	0	0.000%
portals	97.194	97.194	0	0.000%
router	319.612	319.612	0	0.000%
simple_ssr	494.019	494.019	0	0.000%
ssr_router	425.700	425.700	0	0.000%
suspense	110.521	110.521	0	0.000%
timer	89.360	89.360	0	0.000%
todomvc	143.052	143.052	0	0.000%
two_apps	87.286	87.286	0	0.000%
web_worker_fib	153.326	153.326	0	0.000%
webgl	87.373	87.373	0	0.000%

✅ None of the examples has changed their size significantly.