Updated text to reflect the usage as a title with header tag

[kbrandon19]

Description

On the main page there were 3 span tags, within the call-to-action section that were used as a title. I replaced those span tag with h3 tags to match the flow of the site and to make sure it was identified as a header tag for accessebility tools.

Changes were made in the main.css and custom.css to facilitate the changes and styling.

Related Issue

Issue #347

Motivation and Context

This improves accessibility when a voice over tool is used.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

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