rename a bunch of "nch" to "nh".

[kivikakk]

Looks like a copy-paste error from long ago.

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-f49882a	310.8 ± 1.2	307.8	313.1	2.94 ± 0.02
./bench.sh ./comrak-main	311.8 ± 1.0	309.9	314.8	2.95 ± 0.02
./bench.sh ./pulldown-cmark	105.6 ± 0.5	104.1	106.4	1.00
./bench.sh ./cmark-gfm	115.2 ± 1.2	113.2	118.0	1.09 ± 0.01
./bench.sh ./markdown-it	470.7 ± 1.4	468.4	473.9	4.46 ± 0.03

Run on Mon Sep 22 06:22:51 UTC 2025