Fix Sourcehut icon SVG (#58852)

Follow up to https://github.com/zed-industries/zed/pull/58668 — the
previous PR wasn't following the icon design guidelines.

Release Notes:

- N/A

assets/icons/sourcehut.svg

@@ -1 +1,3 @@
-<svg viewBox="0 0 512 512" xmlns="http://www.w3.org/2000/svg"><path d="M256 8C119 8 8 119 8 256s111 248 248 248 248-111 248-248S393 8 256 8zm0 448c-110.5 0-200-89.5-200-200S145.5 56 256 56s200 89.5 200 200-89.5 200-200 200z"></path></svg>
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M8 2C4.68548 2 2 4.68548 2 8C2 11.3145 4.68548 14 8 14C11.3145 14 14 11.3145 14 8C14 4.68548 11.3145 2 8 2ZM8 12.8387C5.32661 12.8387 3.16129 10.6734 3.16129 8C3.16129 5.32661 5.32661 3.16129 8 3.16129C10.6734 3.16129 12.8387 5.32661 12.8387 8C12.8387 10.6734 10.6734 12.8387 8 12.8387Z" fill="#DCE0E5"/>
+</svg>