doc: Add a blank line between paragraph and image

[joshtriplett]

This prevents the large image from appearing inline in the last line of
the paragraph.

Without this fix:

[epage]

Thanks! Must have been a regression from when we moved it to the tutorial as I had looked at t he previous page many times and it didn't look like that.

[coveralls]

Pull Request Test Coverage Report for Build 9736223998

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.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.3%) to 42.012%

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Files with Coverage Reduction	New Missed Lines	%
src/stream/mod.rs	1	25.28%
src/token/mod.rs	1	75.0%
src/macros/seq.rs	2	85.71%

Totals
Change from base Build 9407425121:	0.3%
Covered Lines:	1307
Relevant Lines:	3111

---

💛 - Coveralls