fix: Footer Style 2 Logo is not displayed properly (#40)

What type of PR is this?

/kind bug

What this PR does / why we need it:

修复页脚风格二 Logo 显示不正确的问题。

Which issue(s) this PR fixes:
Fixes [halo-dev/halo#2894](https://github.com/halo-dev/halo/issues/2894)

Does this PR introduce a user-facing change?
```
none
```

templates/modules/footer.html

@@ -52,7 +52,7 @@
       <a href="/" class="mb-6 flex items-center">
         <img
           th:if="${not #strings.isEmpty(theme.config.footer.logo)} or ${not #strings.isEmpty(site.logo)}"
-          th:src="${theme.config.footer.logo ?: site.title}"
+          th:src="${theme.config.footer.logo ?: site.logo}"
           class="mr-3 h-8"
           th:alt="${site.title}"
         />