as stated in https://docs.plone.org/about/license.html, yes you can contribute without signing the Contributor Agreement, but then you have to include with every pull request you make an explicit statement that you agree your contribution can be published under Creative Commons 4.0. A bit annoying, but hey lawyers have to eat too
There’s a legal grey area where spelling corrections and the like are always OK without signing anything or making an explicit statement, because they do not constitute a “work”, and therefore are not under copyright. The problem is, the definition of what is enough to qualify as as a “work” varies from country to country, and is up for legal challenge. So better play it safe.
(note that this is the same for code, by the way. Correcting a bug in code can be seen as not a work, but where’s the limit? What is substantial, what is not? You cannot base this on length alone or other technical measures; the shortest poem in the world consists of one character…)