theming.md Typo Fix (#25875)

it's ≠ its

Author: ethanbeyer

docs/4.0/getting-started/theming.md

@@ -367,7 +367,7 @@ Should you need to modify your `$grid-breakpoints`, your changes will apply to a
 
 ## CSS variables
 
-Bootstrap 4 includes around two dozen [CSS custom properties (variables)](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables) in it's compiled CSS. These provide easy access to commonly used values like our theme colors, breakpoints, and primary font stacks when working in your browser's Inspector, a code sandbox, or general prototyping.
+Bootstrap 4 includes around two dozen [CSS custom properties (variables)](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables) in its compiled CSS. These provide easy access to commonly used values like our theme colors, breakpoints, and primary font stacks when working in your browser's Inspector, a code sandbox, or general prototyping.
 
 ### Available variables