docs(README): fix wrong character in parser example

[palmerye]

Repair the wrong character in Parser config demo.
.sss => .css

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[coveralls]

Coverage remained the same at 89.308% when pulling 9271beb on palmerye:patch-1 into 2484275 on postcss:master.

[alexander-akait]

@palmerye Look on { loader: 'postcss-loader', options: { parser: 'sugarss' } }, sugarss use sss extension. But anyway thanks for PR 👍