Fix method reference in CollectionUtils javadoc

[kirmerzlikin]

Overview

There was an invalid method reference in the javadoc for CollectionUtils.toStream method. This invalid reference caused aggregateJavadocs task to fail, which in turn failed the whole CI build (example - https://github.com/junit-team/junit5/runs/7990513854?check_suite_focus=true).

This PR fixes that invalid method reference.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes
• All continuous integration builds pass

[sormuras]

LGTM

[sormuras]

Thanks for another fix, Kyrylo! Much appreciated.

[sbrannen]

Thanks for the fix!

I've back ported this to 5.9.x for inclusion in 5.9.1.