Adjust erf estimation function (#15423)

Release Notes:

- Fixed a (potential) small error in erf estimation. Technically, the
error is negligible.

I am not sure where the current calculation for erf come from and if it
is intended or a simple mistake. However it looks slightly different
from the official calculation, notably
[this](https://en.wikipedia.org/wiki/Error_function#Approximation_with_elementary_functions)
from Wikipedia.

I will add a comment if it is intended.

crates/gpui/examples/shadow.rs

@@ -0,0 +1,29 @@
+use gpui::*;
+
+struct Shadow {}
+
+impl Render for Shadow {
+    fn render(&mut self, _cx: &mut ViewContext<Self>) -> impl IntoElement {
+        div()
+            .flex()
+            .bg(rgb(0xffffff))
+            .size_full()
+            .justify_center()
+            .items_center()
+            .child(div().size_8().shadow_sm())
+    }
+}
+
+fn main() {
+    App::new().run(|cx: &mut AppContext| {
+        let bounds = Bounds::centered(None, size(px(300.0), px(300.0)), cx);
+        cx.open_window(
+            WindowOptions {
+                window_bounds: Some(WindowBounds::Windowed(bounds)),
+                ..Default::default()
+            },
+            |cx| cx.new_view(|_cx| Shadow {}),
+        )
+        .unwrap();
+    });
+}