validating atom feed

On validating atom feed of my site, I find that the feedvalidator.com site:

http://feedvalidator.org/check.cgi?url=http://www.i3pep.org/feed/atom/

gives the following kind of error:
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This feed is valid, but may cause problems for some users. We recommend fixing these problems.
line 18, column 31: Identifier “http://www.i3pep.org?p=72” is not in canonical form (should be “http://www.i3pep.org/?p=72”)
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is there any solution – how to rectify it? thanks.