Stop steps list regex matching newlines

The regex used `^\s*` to force the match to be anchored to the start of a line followed by whitespace, but `\s` matches newlines as well as tabs and spaces which causes the replaced markup to not be separated from the content above by a double newline. See test for an example.

This change makes the regex use `[ \t]` to match acceptable whitespace for the start of the line.