runtime(dosini): Update syntax script, spellcheck comments only

runtime(dosini): Update syntax script, spellcheck comments only

By default spell checking is enabled for all text, but adding
`contains=@Spell` to syntax rules restricts spell checking to those
syntax rules.  See `:help spell-syntax` for full details.

Variable names and headers are far more likely than comments to contain
spelling errors, so only enable spell checking in comments.

Introduced in https://github.com/xuhdev/syntax-dosini.vim/pull/8

cc @tobinjt

closes: #15655

Signed-off-by: John Tobin <johntobin@johntobin.ie>
Signed-off-by: Hong Xu <hong@topbug.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>