bug fix: the configure option string was misspelled

[[Imported from SVN: r5445]]

m4/dune_exprtmpl.m4

 AC_DEFUN([DUNE_EXPRTMPL],[
   # enable experimental features
   AC_ARG_ENABLE(expressiontemplates,
-   AC_HELP_STRING([--enable-exmpressiontemplates],[enable experimental expressiontemplates in dune]))
+   AC_HELP_STRING([--enable-expressiontemplates],[enable experimental expressiontemplates in dune]))
   if test x$enable_expressiontemplates = xyes; then
     AC_DEFINE([DUNE_EXPRESSIONTEMPLATES], [1], 
       [Define to 1 if the experimental expression templates should be used])