fix(marketing): hero counter uses shorthand count += 1 (#3)

Marketing pitch advertises 'wrap a value in wire() and any template
that reads it updates automatically' — keep the snippet to the
shorthand, not .value.

Author: reecelikesramen

site/src/pages/index.astro

@@ -21,7 +21,7 @@ const pywireCode = `---
 count = wire(0)
 ---
 <p>Count: {count}</p>
-<button @click={count.value += 1}>
+<button @click={count += 1}>
     Add One
 </button>`