Update rustdoc test following DOM change

Author: GuillaumeGomez

src/test/rustdoc/stability.rs

@@ -4,7 +4,7 @@
 
 pub struct Unstable {
     // @has stability/struct.Unstable.html \
-    //      '//div[@class="item-info"]//div[@class="stab unstable"]' \
+    //      '//span[@class="item-info"]//div[@class="stab unstable"]' \
     //      'This is a nightly-only experimental API'
     // @count stability/struct.Unstable.html '//span[@class="stab unstable"]' 0
     pub foo: u32,