Quote \[cmd] in docstring of org-export-with-LaTeX-fragments

* org-exp.el (org-export-with-LaTeX-fragments): Fix docstring so that
\\[ a = b \\] is not interpreted as a keybinding by
`substitute-command-keys'.

In a docstring \\[text] is used to indicate that text should be
interpreted by `substitute-command-keys'.  To avoid this
interpretation, we need to prefix it with \\=.

lisp/org-exp.el

@@ -475,8 +475,8 @@ This option can also be set with the +OPTIONS line, e.g. \"TeX:nil\"."
   "Non-nil means process LaTeX math fragments for HTML display.
 When set, the exporter will find and process LaTeX environments if the
 \\begin line is the first non-white thing on a line.  It will also find
-and process  the math delimiters like $a=b$ and \\( a=b \\) for inline math,
-$$a=b$$ and \\[ a=b \\] for display math.
+and process the math delimiters like $a=b$ and \\( a=b \\) for inline math,
+$$a=b$$ and \\=\\[ a=b \\] for display math.
 
 This option can also be set with the +OPTIONS line, e.g. \"LaTeX:mathjax\".