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MINOR: [CI] Remove needless ":" from Dependabot PR subject (#36216)

### Rationale for this change

    Before: "MINOR: [CI]: Bump ..."
    After:  "MINOR: [CI] Bump ..."

The "After" version is our style.

### What changes are included in this PR?

Add " " to `prefix:`:

https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#commit-message

> if you end the prefix with a whitespace, there will be no colon
> added between the prefix and the commit message.

### Are these changes tested?

No. Sorry.

### Are there any user-facing changes?

No.

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>