Fix PEG parsers' "comment" rule

[smurfix]

The problem with the previous version was that if there is no text after the '//', regular whitespace processing kicked in and the regexp was applied to the next non-empty line.

The effect of this was that the next line gets commented off unconditionally, which is kindof not what you want.

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• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
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• Tests have been included and/or updated
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[igordejanovic]

Looks good. Thanks.