Help: Fix version added for `CMAKE_MATCH_<n>`

This was added in 2.6.0 by commit c1b3484c2d (ENH: store the matches for
paren-delimited subexpression..., 2007-08-21, v2.6.0~1227), not in 3.9.

Only the documentation at this place was added in 3.9, by commit
eb08e1febb (Help: Document CMAKE_MATCH_<n> variables, 2017-04-18,
v3.9.0-rc1~257^2)

Help/command/if.rst

@@ -264,7 +264,7 @@ Comparisons
   True if the given string or variable's value matches the given regular
   expression.  See :ref:`Regex Specification` for regex format.
 
-  .. versionadded:: 3.9
+  .. versionadded:: 2.6
    ``()`` groups are captured in :variable:`CMAKE_MATCH_<n>` variables.
 
 .. signature:: if(<variable|string> LESS <variable|string>)