Remove duplicate return type in create_redirect_url spec

[Nezteb]

While using oidcc, I noticed that one of the return types is duplicated. I wouldn't really define it as a bug, so I labelled it as an "improvement". 😄

[maennchen]

Thanks @Nezteb ❤️

[coveralls]

Pull Request Test Coverage Report for Build 191

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.6%) to 91.5%

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Files with Coverage Reduction	New Missed Lines	%
src/oidcc_jwt_util.erl	2	79.33%
src/oidcc_token.erl	5	88.66%

Totals
Change from base Build 178:	-0.6%
Covered Lines:	1055
Relevant Lines:	1153

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💛 - Coveralls