CSS Units lesson: Fix a simple grammar error

[devritra]

fixed the grammar at Relative units -> Viewport units section

Because

This PR is my first contribution. I have done it as a hands on practice.
It fixes a simple grammatical error which will enhance the reading experience even if it's a small improvement.

This PR

• The change is done at Relative units -> Viewport units section
• Changed "examples including full-height heroes, full-screen app-like interfaces." to "examples including full-height heroes and full-screen app-like interfaces."

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
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