perf: improve the text of the upgrade theme button (#747)

#### What type of PR is this?

/kind bug

#### What this PR does / why we need it:

修改主题升级文案

#### Which issue(s) this PR fixes:

Fixes https://github.com/halo-dev/halo/issues/2857

#### Does this PR introduce a user-facing change?

```release-note
None
```

src/modules/interface/themes/ThemeDetail.vue

@@ -95,7 +95,7 @@ const onUpgradeModalClose = () => {
                     type="secondary"
                     @click="upgradeModal = true"
                   >
-                    更新
+                    升级
                   </VButton>
                   <VButton
                     v-close-popper