Fix UI (#13239)

### What problem does this PR solve?

This pull request makes a minor update to the English locale strings for
the Table of Contents toggle buttons, changing the labels from "Show
TOC"/"Hide TOC" to "Show content"/"Hide content" for improved clarity.

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

web/src/locales/en.ts

@@ -1324,8 +1324,8 @@ Example: Virtual Hosted Style`,
         image2text: 'OCR',
         speech2text: 'ASR',
       },
-      showToc: 'Show TOC',
-      hideToc: 'Hide TOC',
+      showToc: 'Show content',
+      hideToc: 'Hide content',
     },
     message: {
       registered: 'Registered!',