Use https scheme

[muzimuzhi]

Convert http to https scheme for all links in the repo, including examples in spec.txt (not sure if welcome).

The remaining http schemes are

• a link to www.aaronsw.com in spec.txt because it doesn't support https,
• all links in changelog.txt because I assume released changelog entries should be kept as-is, and
• a link in CommonMark.dtd because I know nothing about .dtd file format.

# need GNU grep to use option "-P"
grep --color=auto --exclude-dir=.git -rnP '(?<!/)http://' .
./spec.txt:1142:[original ATX implementation](http://www.aaronsw.com/2002/atx/atx.py),
./changelog.txt:311:    Discussion at <http://talk.commonmark.org/t/555>.
./changelog.txt:314:    Discussion at <http://talk.commonmark.org/t/779> and
./changelog.txt:315:    <http://talk.commonmark.org/t/1287/12>.
./changelog.txt:346:    See http://talk.commonmark.org/t/horizontal-rule-or-thematic-break/912/3
./changelog.txt:503:    <http://talk.commonmark.org/t/odd-list-behaviour/1189>).
./changelog.txt:624:    <http://talk.commonmark.org/t/903/6>. The basic idea of the change
./CommonMark.dtd:10:    xmlns CDATA #FIXED "http://commonmark.org/xml/1.0">

[wooorm]

it might be nice to still have one test that uses http:. Otherwise looks good

[muzimuzhi]

@wooorm Now example 593, the example right after doc for scheme, uses both http and https.

[jgm]

I would suggest just using http in that example -- after all, the next example uses https.

[muzimuzhi]

I'm a little worried that using http solely in a single example might look like it's missed from a http->https substitution.

after all, the next example uses https.

Ok, I'm convinced.