Modal Dialog Example: Change dialog title elements from H2 to H1 (pull #3412)

Fixes #3395 by changing the dialog title elements from H2 to H1.

When the dialog example code included H2 elements for the dialog titles, the script that builds the table of contents for the example page was including the dialog titles in the table of contents, which was confusing for readers because the dialog titles are not visible unless the reader operates the dialog example.