org.texi: Fix un-exportable table

* doc/org.texi (Column groups): Prevent "[4]" from being interpreted
  as a footnote reference so as to make table exportable.

Reported-by: Nicolas Richard <youngfrog@members.fsf.org>
<http://permalink.gmane.org/gmane.emacs.orgmode/98816>

doc/org.texi

@@ -2392,13 +2392,13 @@ a group of its own.  Boundaries between column groups will upon export be
 marked with vertical lines.  Here is an example:
 
 @example
-| N | N^2 | N^3 | N^4 | sqrt(n) | sqrt[4](N) |
-|---+-----+-----+-----+---------+------------|
-| / |   < |     |   > |       < |          > |
-| 1 |   1 |   1 |   1 |       1 |          1 |
-| 2 |   4 |   8 |  16 |  1.4142 |     1.1892 |
-| 3 |   9 |  27 |  81 |  1.7321 |     1.3161 |
-|---+-----+-----+-----+---------+------------|
+| N | N^2 | N^3 | N^4 | ~sqrt(n)~ | ~sqrt[4](N)~ |
+|---+-----+-----+-----+-----------+--------------|
+| / |   < |     |   > |         < |            > |
+| 1 |   1 |   1 |   1 |         1 |            1 |
+| 2 |   4 |   8 |  16 |    1.4142 |       1.1892 |
+| 3 |   9 |  27 |  81 |    1.7321 |       1.3161 |
+|---+-----+-----+-----+-----------+--------------|
 #+TBLFM: $2=$1^2::$3=$1^3::$4=$1^4::$5=sqrt($1)::$6=sqrt(sqrt(($1)))
 @end example