parameter updates in documentation

[heckj]

While using this package as test subject while debugging another issue, I noticed a few warnings about misnamed or missing parameters (thank you latest DocC diagnostics!). This updates those to resolve the doc warnings (and correct the documentation)

• resolved warnings generated by latest DocC compilation that
  highlighted missing, renamed, or misnamed parameters within
  the documentation.

[Kyle-Ye]

@swift-ci test

[Kyle-Ye]

LGTM

[Kyle-Ye]

I see there are still other warnings. But this looks good to me so far.

warning: 'Snippets' is ambiguous at '/Markdown'
  --> Markdown.md:17:8-17:16
15 | A quick overview of examples showing tasks you can achieve with Swift Markdown.
16 |
17 + - <doc:Snippets>
   |                ├─suggestion: Insert 'anchor' for'Snippets'
   |                ╰─suggestion: Insert 'article' for'Snippets'
18 |
19 | ### Getting Started

warning: 'Infrastructure' is ambiguous at '/Markdown'
  --> Markdown.md:44:8-44:22
42 | ### Infrastructure
43 |
44 + - <doc:Infrastructure>
   |                      ├─suggestion: Insert 'anchor' for'Infrastructure'
   |                      ╰─suggestion: Insert 'article' for'Infrastructure'
45 |
46 | ### Visit MarkupFinished building documentation for 'Markdown' (1.19s)