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 <[e-mail]>
Signed-off-by: WangYuli <[e-mail]>
Reviewed-by: Yanteng Si <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/strparser.rst

···
     struct contains two fields: offset and full_len. Offset is
     where the message starts in the skb, and full_len is the
     the length of the message. skb->len - offset may be greater
-    then full_len since strparser does not trim the skb.
+    than full_len since strparser does not trim the skb.
 
     ::