[css-borders-4][editorial] Fix syntax rewrite mistake

[cdoublev]

The syntax of corner-top-left and other "single corner" shorthands is:

Value: <'border-top-left-radius'>{1,2} || <'corner-top-left-shape'>

{1,2} is an oversight, because the syntax of border-top-left-radius is:

Value: <length-percentage [0,∞]>{1,2}

Before 5b61925, he syntax of corner-top-left and other "single corner" shorthands was:

Value: <length-percentage [0,&infin;]>{1,2} || <corner-shape-value>

My apologies.

[noamr]

LGTM