Fix missing tuple in code example

[JonathanPlasse]

Hi,
Just started using nom and I am loving it!
Thank you for your awesome work!

I think tuple is missing in the code example.

[Geal]

thanks!

[coveralls]

Pull Request Test Coverage Report for Build 8774141498

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.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• 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.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.1%) to 62.342%

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Files with Coverage Reduction	New Missed Lines	%
src/lib.rs	1	66.67%
src/error.rs	4	16.41%

Totals
Change from base Build 6790049741:	0.1%
Covered Lines:	1826
Relevant Lines:	2929

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💛 - Coveralls