Fix incorrect setext heading source span

[artempyanykh]

Previously, heading's span would be 3-6; where 6 is outside the bounds of the original string.

For the context, the original bug report in Marksman #156 (I use markdig there).

[coveralls]

Pull Request Test Coverage Report for Build 4152367422

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

• 8 of 8 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 93.293%

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Totals
Change from base Build 3993718076:	0.001%
Covered Lines:	25923
Relevant Lines:	27178

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💛 - Coveralls

[MihaZupan]

I'm guessing you didn't want to close this one @artempyanykh :)

[artempyanykh]

Haha, no, not really 🙂 Thanks for catching this @MihaZupan!

[xoofx]

Thanks!