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GH-37213: [C#] Updating a reference to FlatBuffers missed due to rebase/merge conflict (#37214)

### Rationale for this change

The build was broken

### What changes are included in this PR?

Updated a newly introduced reference to `using FlatBuffers` to be `using Google.FlatBuffers`

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* Closes: #37213

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>