Syntax documenttation on thematic breaks contridicted by test

[leroycep]

The syntax documentation for thematic breaks says:

A line containing four or more * or - characters, and nothing else (except spaces or tabs) is treated is a thematic break (<hr> in HTML).

I interpreted this to mean four characters, excluding spaces.

However the first test in the thematic breaks file contradicts this:

djot/test/thematic_breaks.test

Lines 1 to 11 in 47f9b3b

	```
	hello
	- - -
	there
	.
	<p>hello</p>
	<hr>
	<p>there</p>
	```

Is the count supposed to include spaces, or should it only include * and - characters?

[jgm]

I've updated the docs to say "three or more," which is what the code says.

A case could be made for requiring four or more -- the case would be that --- is already an em-dash. However, that problem could also be solved by just requiring * characters for thematic breaks (as someone has already suggested on another issue).