[css-display] link to css-position for 'fixed'

[ewilligers]

This avoid the blikeshed warning

LINK ERROR: Multiple possible 'fixed' maybe refs.
Arbitrarily chose https://drafts.csswg.org/css-position-3/#valdef-position-fixed
To auto-select one of the following refs, insert one of these lines into a

 block:

spec:css-backgrounds-3; type:value; text:fixed

spec:css-counter-styles-3; type:value; text:fixed

spec:css-position-3; type:value; text:fixed

spec:css-device-adapt-1; type:value; text:fixed

[fantasai]

This seems to have been fixed by specifying ''position/fixed'' in the source already.

[ewilligers]

Fixed by 65ba881