Fix incorrect smart quotes

[kaushalmodi]

This Org:

** Footnotes 1
:PROPERTIES:
:EXPORT_DATE: 2017-07-21
:EXPORT_FILE_NAME: footnotes-1
:END:
This is some text[fn:1].

/Note to self: You *cannot* name an Org heading 'Footnotes'; that's
reserved by Org to store all the footnotes./
* Footnotes
[fn:1] First footnote

converts to this Markdown:

This is some text[^fn:1].

_Note to self: You **cannot** name an Org heading ’Footnotes’; that’s
reserved by Org to store all the footnotes._

[^fn:1]: First footnote

Note that right-single quotes are used on both sides of Footnotes here: ’Footnotes’;.

Disabling smart quotes in Org export fixes this.