Fix grammatical error in mask-mode description (mdn#43351)

Corrected grammatical error in the description section (it's -> its)

Author: jmanj78

files/en-us/web/css/reference/properties/mask-mode/index.md

@@ -44,7 +44,7 @@ The `mask-mode` property can take multiple comma-separated `<masking-mode>` keyw
 
 ## Description
 
-A mask transfers its transparency, and depending on the mask type, it's luminance, to the element it is masking.
+A mask transfers its transparency, and depending on the mask type, its luminance, to the element it is masking.
 If the mask is of type {{cssxref("image")}}, by default the alpha values of the mask image determine the visibility of each part of the masked element: where the mask is opaque, the corresponding part of the masked element is visible; where the mask is translucent, the element is as well, with those areas of the element being hidden. This is the default behavior for `<image>` masks when the `mask-mode` property is set to or defaults to `match-source`, and it is always the case when the `mask-mode` is explicitly set to `alpha`.
 
 ### Understanding luminance