docs: networking: strparser: Fix a typo

docs: networking: strparser: Fix a typo

The context indicates that 'than' is the correct word instead of 'then',
as a comparison is being performed.

Given that 'then' is also a valid English word, checkpatch.pl wouldn't
have picked up on this spelling error.

This typo was caught by AI during code review.

Suggested-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: WangYuli <wangyuli@uniontech.com>
Reviewed-by: Yanteng Si <si.yanteng@linux.dev>
Link: https://patch.msgid.link/A43BEA49ED5CC6E5+20250318074656.644391-1-wangyuli@uniontech.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>