Fix example in docs for sam_hdr_add_line

[kojix2]

Hi!

I am glad to have the opportunity to send you another pull request.
In the example in the sam_hdr_add_line documentation, I propose changing "ID" to "SN."

[E::sam_hrecs_update_hashes] Header includes @SQ line with no SN: tag

htslib/htslib/sam.h

Line 471 in 279cc9e

* @param lines Full SAM header record, eg "@SQ\tSN:foo\tLN:100", with

Best regards.

[daviesrob]

Oops, yes, that example was wrong. Thanks for the fix.