docs: Use correct Valadoc syntax for monospaced content

docs: Use correct Valadoc syntax for monospaced content

The syntax is ‘``monospaced text``’ rather than ‘`monospaced text`’, which
we were consistently getting wrong. Valadoc now produces the correct markup
for monospaced text, but it doesn’t appear differently from other text at
the moment due to Valadoc not having any CSS for it. That will change soon.

Similarly, Valadoc currently produces the wrong markup for monospaced text
with the gtkdoc doclet, but that will soon be fixed.

See: https://bugzilla.gnome.org/show_bug.cgi?id=681721