ox-latex: Fix docstring

* lisp/ox-latex.el (org-latex-listings-options): Use correct number of
  backslash characters in the example.

Reported-by: Thomas S. Dye <tsd@tsdye.com>

lisp/ox-latex.el

@@ -690,8 +690,8 @@ a list containing two strings: the name of the option, and the
 value.  For example,
 
   (setq org-latex-listings-options
-    '((\"basicstyle\" \"\\small\")
-      (\"keywordstyle\" \"\\color{black}\\bfseries\\underbar\")))
+    '((\"basicstyle\" \"\\\\small\")
+      (\"keywordstyle\" \"\\\\color{black}\\\\bfseries\\\\underbar\")))
 
 will typeset the code in a small size font with underlined, bold
 black keywords.