Update tutorial-define-index.asciidoc (#76975) (#78621)

* Update tutorial-define-index.asciidoc

Forgot to update alt text in previous pr. Additionally, it is unclear in the image where the "time field" dropdown is located.

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

Co-authored-by: Kaarina Tungseth <kaarina.tungseth@elastic.co>

Co-authored-by: ncheckin <68351161+ncheckin@users.noreply.github.com>

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

@@ -45,6 +45,6 @@ contains the time series data.
 . From the *Time field* dropdown, select *@timestamp, then click *Create index pattern*.
 +
 [role="screenshot"]
-image::images/tutorial_index_patterns.png[All tutorial index patterns]
+image::images/tutorial_index_patterns.png[Image showing how to create an index pattern]