doc: fix rare case of misaligned columns

By using the same unit for the offset of the main column as used for the
left column width, we ensure that browsers render the columns
conistently.

Ref: https://github.com/nodejs/help/issues/32

PR-URL: https://github.com/nodejs/node/pull/3948


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>