Fix curved quotes in error message

* lisp/org.el (org-create-formula-image): Use back-quote + single
  quote to get the correct rendering of curved quotes.

Message displayed before this fix: "Please adjust ’imagemagick’ .."

Message displayed after this fix:  "Please adjust ‘imagemagick’ .."

The effect of this fix is visible on emacs 25.1 and newer (See Emacs
NEWS).

lisp/org.el

@@ -19462,7 +19462,7 @@ a HTML file."
 	(insert latex-header)
 	(insert "\n\\begin{document}\n" string "\n\\end{document}\n")))
 
-    (let* ((err-msg (format "Please adjust '%s' part of \
+    (let* ((err-msg (format "Please adjust `%s' part of \
 `org-preview-latex-process-alist'."
 			    processing-type))
 	   (image-input-file