README: sync to sample (and improve sample quality).

[kivikakk]

The README examples haven't actually worked since #601. They're also kinda dry. Fix both.

Thanks @Turbo87 for pointing this out! (#601 (comment))

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-540ca52	310.4 ± 4.1	307.1	328.8	2.96 ± 0.04
./bench.sh ./comrak-main	313.1 ± 11.7	306.1	347.8	2.99 ± 0.11
./bench.sh ./pulldown-cmark	104.9 ± 0.4	104.2	106.0	1.00
./bench.sh ./cmark-gfm	116.1 ± 6.4	112.3	139.9	1.11 ± 0.06
./bench.sh ./markdown-it	478.2 ± 7.6	471.5	500.9	4.56 ± 0.07

Run on Tue Oct 14 06:35:55 UTC 2025