(Tests) Switch es_MX examples to use lower-case month names

(Tests) Switch es_MX examples to use lower-case month names

There is a discrepancy in how month-names are capitalized, depending on the
source of translation. `ts()` tends toward uppercase, and `IntlDateFormatter`
tends toward lowercase.

Since I don't know the capitalization rules for `es_MX`, I checked two sources:

* Asked a bilingual speaker (native `es_MX`), and she went back-and-forth.
  Her first reaction was uppercase, but (after searching) she settled on
  lowercase.

* Asked `translate.google.com` for a few phrases ("John was born on the
  first Thursday of March." and "Today is July 2nd" and "September is the
  best month.").  `Spanish` seemed to work like `French` -- using the
  lowercase for month and day-of-week (except when handling the
  first-word-of-sentence).

It seems like lowercase is better for `es_MX`, so this patch switches the
examples to lowercase.  Incidentally, the `es_MX` examples now resemeble the
`fr_FR` examples, and they match `IntlDateFormatter`.