Fixed ordered list markdown renderer.

[BobMarlon]

Hi,
first of all, thanks for creating this library! That was just what I needed right now. I stumbled over a bug however, which caused ordered lists to not be enumerated correctly if rendered using the MarkdownRenderer.

This can be reproduced like that:

import marko
from marko.md_renderer import MarkdownRenderer

md = marko.Markdown(renderer=MarkdownRenderer)
doc = md.parse("""
1. a
2. b
3. c
""")
print(md.render(doc))

Prints:

1. a
1. b
1. c

The fix is kinda hacky though. I also didn't add tests...

[frostming]

fwiw the current output is also a valid markdown and can be rendered rightly on most markdown renderer, not a bug.

what doesn't work well?

[BobMarlon]

Well, I guess it depends on how you look at it. The output may be valid but If you take a look at the documentation in the MarkdownRenderer class:

It is useful for, e.g. merging sections and formatting documents.

I would have expected that if I simply parse a markdown document without doing anything and then render it again into markdown, that I get the same as in the beginning. This is why I noticed that because formatting documents was my use case.

[codecov]

Codecov Report

Merging #112 (3249929) into master (a8997f0) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   95.56%   95.56%   -0.01%     
==========================================
  Files          19       19              
  Lines        1781     1780       -1     
  Branches      289      289              
==========================================
- Hits         1702     1701       -1     
  Misses         42       42              
  Partials       37       37              

Flag	Coverage Δ
unittests	95.56% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
marko/md_renderer.py	100.00% <100.00%> (ø)

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