Fix typo in non-text-contrast.html (#3705)

Fixed typo

Closes #3704

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Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>

understanding/21/non-text-contrast.html

@@ -318,7 +318,7 @@ <h3>Graphical Objects</h3>
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 							<td><img src="img/contrast-phone.png" alt="An orange circle with a white telephone icon in the middle." width="40"/></td>
-							<td><p>The phone icon is a simple shape within the orange (#E3660E) circle. The meaning can be understood from that icon alone, the background behind the circle is irrelevant. The orange background and the white icon have a contrast ration greater than 3:1, which passes.</p>
+							<td><p>The phone icon is a simple shape within the orange (#E3660E) circle. The meaning can be understood from that icon alone, the background behind the circle is irrelevant. The orange background and the white icon have a contrast ratio greater than 3:1, which passes.</p>
 							<p>The graphical object is the white phone icon.</p>
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 						</tr>