tzfile.5: s/only/& if/ (thanks to Andreas Schwab)

tzfile.5

@@ -259,7 +259,7 @@ the lowest version number needed by a file's data.
 For example, a writer should generate a version 4 file
 only if its leap second table either expires or is truncated at the start.
 Likewise, a writer not generating a version 4 file
-should generate a version 3 file only
+should generate a version 3 file only if
 TZ string extensions are necessary to accurately
 model transition times.
 .PP