Add missing translation for the filter-epression field placeholder

[krakan]

Description

Add the {% trans %}/{% endtrans %} tags for the placeholder text of the filter-expression field on the search.html page.

Motivation and Context

Needed in order for translators to be able to translate it to other languages.

Screenshots (if appropriate):

Types of changes

• Replay fix (fixes a replay specific issue)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added or updated tests to cover my changes.
• All new and existing tests passed.

The tests/test_auto_colls.py::TestManagedColls::test_add_static test is failing with and without this change. The other 807 tests passed.