autotools: fix the name of the --with-unicode-version configure option

configure.ac

@@ -24,7 +24,7 @@ dnl UCD data configuration.
 dnl ================================================================
 
 AC_ARG_WITH([unicode-version],
-    [AS_HELP_STRING([--unicode-version], [Unicode version to support @<:@default=6.2.0@:>@])],
+    [AS_HELP_STRING([--with-unicode-version], [Unicode version to support @<:@default=6.2.0@:>@])],
     [AS_IF([test x"$withval" != x],
            [UCD_VERSION="$withval"])],
     [UCD_VERSION="6.2.0"])