Add missing hyperlink to the tuple combinator in doc

[jmmaa]

hyperlink is missing in docs, specifically in choosing_a_combinator.md

[Geal]

thanks!

[coveralls]

Pull Request Test Coverage Report for Build 4149320146

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.08%) to 62.135%

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Files with Coverage Reduction	New Missed Lines	%
src/character/complete.rs	1	81.12%
src/character/streaming.rs	1	78.38%
<!--	Total:	2		-->

Totals
Change from base Build 3981619336:	-0.08%
Covered Lines:	1554
Relevant Lines:	2501

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💛 - Coveralls