Update date pattern in tutorial-define-index (elastic#9808)

Backports PR elastic#9806

In `tutorial-define.text.asciidoc`, it is stated that the typical index name contains the date as `MM-DD-YYYY`, and then proceeds to give an example of how a log index from May, 2015 would look like: `logstash-2015.05*`, which doesn't match the typical pattern for dates stated above.

This PR changes the date pattern, as to me `YYYY.MM.DD` is more accurate on a world-wide scale.

Author: elastic-jasper

docs/getting-started/tutorial-define-index.asciidoc

@@ -4,7 +4,7 @@
 Each set of data loaded to Elasticsearch has an index pattern. In the previous section, the
 Shakespeare data set has an index named `shakespeare`, and the accounts  data set has an index named `bank`. An _index
 pattern_ is a string with optional wildcards that can match multiple indices. For example, in the common logging use
-case, a typical index name contains the date in MM-DD-YYYY format, and an index pattern for May would look something
+case, a typical index name contains the date in YYYY.MM.DD format, and an index pattern for May would look something
 like `logstash-2015.05*`.
 
 For this tutorial, any pattern that matches the name of an index we've loaded will work. Open a browser and