Bids: update statistic.value description

[odscjen]

Follow on from open-contracting-extensions/ocds_participationFee_extension#27 (comment)

Change

The value for the measure in question. Total counts should be provided as an integer. Percentages should be provided as a proportion of 1 (e.g. 10% = 0.1)

to

The value for the measure in question. A total count should be expressed as an integer. A proportion should be expressed as a decimal (for example, 0.1) and not as a percentage (for example, 10).