The markup for a citation is double question marks. The em dash character is part of the rendering.
??citation??
For instructions, refer to the Atlassian help page:
https://confluence.atlassian.com/conf613/confluence-wiki-markup-964960693.html 
The double question marks are rendered as follows in the XHTML:
<cite></cite>
If you want the citation to be inline, rather than a separate paragraph, you can remove the paragraph breaks around the citation, in the Source Editor.
