Fix possessive "its" in docs and comments (#3998)

This PR fixes a number of places where we were incorrectly using "it's"
where we needed to use the possessive "its".

Release Notes:

- N/A

crates/gpui/src/action.rs

@@ -104,7 +104,7 @@ pub struct ActionData {
 }
 
 /// This constant must be public to be accessible from other crates.
-/// But it's existence is an implementation detail and should not be used directly.
+/// But its existence is an implementation detail and should not be used directly.
 #[doc(hidden)]
 #[linkme::distributed_slice]
 pub static __GPUI_ACTIONS: [MacroActionBuilder];

crates/gpui/src/color.rs

@@ -321,7 +321,7 @@ impl Hsla {
     ///
     /// Assumptions:
     /// - Alpha values are contained in the range [0, 1], with 1 as fully opaque and 0 as fully transparent.
-    /// - The relative contributions of `self` and `other` is based on `self`'s alpha value (`self.a`) and `other`'s  alpha value (`other.a`), `self` contributing `self.a * (1.0 - other.a)` and `other` contributing it's own alpha value.
+    /// - The relative contributions of `self` and `other` is based on `self`'s alpha value (`self.a`) and `other`'s  alpha value (`other.a`), `self` contributing `self.a * (1.0 - other.a)` and `other` contributing its own alpha value.
     /// - RGB color components are contained in the range [0, 1].
     /// - If `self` and `other` colors are out of the valid range, the blend operation's output and behavior is undefined.
     pub fn blend(self, other: Hsla) -> Hsla {

crates/gpui/src/elements/overlay.rs

@@ -45,7 +45,7 @@ impl Overlay {
     }
 
     /// Sets the position mode for this overlay. Local will have this
-    /// interpret it's [Overlay::position] as relative to the parent element.
+    /// interpret its [`Overlay::position`] as relative to the parent element.
     /// While Window will have it interpret the position as relative to the window.
     pub fn position_mode(mut self, mode: OverlayPositionMode) -> Self {
         self.position_mode = mode;