Print list without space in front of *

I noticed, that round tripping markdown adds spaces in front of `*`:
```julia
marky = md"""
# Test

* test test
"""
string(marky)
```
Results in:
```
"# Test\n\n  * test test\n"
```
I guess there are no round tripping guarantees for this, but I think this is not what a markdown formatter would do?
I'm not sure what `plain` wants to achieve here with the space, so I wanted to put this up for discussion via this PR! 
Not really important in general, but for my use case it would be nice to drop the space ;)

Author: SimonDanisch

stdlib/Markdown/src/render/plain.jl

@@ -35,7 +35,7 @@ end
 
 function plain(io::IO, list::List)
     for (i, item) in enumerate(list.items)
-        print(io, isordered(list) ? "$(i + list.ordered - 1). " : "  * ")
+        print(io, isordered(list) ? "$(i + list.ordered - 1). " : "* ")
         lines = split(rstrip(sprint(plain, item)), "\n")
         for (n, line) in enumerate(lines)
             print(io, (n == 1 || isempty(line)) ? "" : "    ", line)