[fix] correct error message for staggered grid overlaps

When one wants to run simulations in parallel using the staggered grid method, when using an Overlap of greater than 1, the condition in dumux/discretization/checkoverlapsize.hh called in the function isValid() in line 58 will return false, as it requires an overlap of exactly 1. However, the error message that will show up in the command line reads, that an overlap of at least 1 is required. This fix changes the error message to avoid further confusion.