Fix broken bn_BD placeholder

Running dennis locally showed the following error.

```
$ dennis-cmd lint --errorsonly locale/
dennis version 0.9
>>> Working on: /home/whatever/code/addons-frontend/locale/bn_BD/LC_MESSAGES/amo.po
E201: invalid variables: {query}
526:#: src/amo/components/SearchContextCard/index.js:38
527:msgid "Searching for \"%(query)s\""
528:msgstr "\"{query}\" এর জন্য অনুসন্ধান"
```

