Update paragraph block placeholder text format for consistency

[dhananjaykuber]

Fixes #56371

What?

Updates placeholder text in paragraph blocks to use ' / ' for the forward slash key.

Why?

Fixes confusion for new users who misinterpreted "Type / to choose a block" as "Type OR choose a block"

How?

Updated the placeholder text to 'Type ' / ' to choose a block'

Testing Instructions

1. Open block editor
2. Create new paragraph block
3. Verify placeholder shows ' / ' in placeholder

Screenshots

Before

After

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