Correct format string used by DateTimeConverter for invariant culture

[stakx]

Summary

The Remarks section of the API reference docs for System.ComponentModel.DateTimeConverter contains a very minor inaccuracy, stating that this converter uses dddd-mm-yy as the format when formatting DateTimes for the invariant culture.

This is incorrect, the actual used format is dddd-MM-yy. (Note the capitalized MM, which stands for months, whereas lower-case mm would designate minutes.)

[rpetrusha]

Thanks you for contributing to the dotnet/dotnet-api-docs repo, @stakx, and for correcting this incorrect custom format string. We'll merge your PR once our internal build completes successfully.