The CommonMark spec shows that it's not necessary to have a space
between the code fence and the language string:
https://spec.commonmark.org/0.29/#example-112
This also avoids a needless trailing space after a code fence that
does not include a language string.
Change-Id: I2addd38a196045a7442150760b73269bfe4ffb22
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>