CSS Units: Fix grammar (#30929)

fixed the grammar at Relative units -> Viewport units section

Author: devritra

intermediate_html_css/intermediate_css_concepts/css_units.md

@@ -29,7 +29,7 @@ Using a relative size like `rem` to define font sizes across your website *is* r
 
 #### Viewport units
 
-The units `vh` and `vw` relate to the size of the viewport. Specifically, `1vh` is equal to `1%` of the viewport height and `1vw` is equal to `1%` of the viewport width. These can be useful any time you want something to be sized relative to the viewport, examples including full-height heroes, full-screen app-like interfaces.
+The units `vh` and `vw` relate to the size of the viewport. Specifically, `1vh` is equal to `1%` of the viewport height and `1vw` is equal to `1%` of the viewport width. These can be useful any time you want something to be sized relative to the viewport, examples including full-height heroes and full-screen app-like interfaces.
 
 ### So many units!