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MINOR: [Java] Fix develocity cache directory name in .gitignore (#41866)

### Rationale for this change

`.gitignore` is not referencing correctly the develocity cache directory

### What changes are included in this PR?

Changing from `.mvn/.develocity.xml` to `.mvn/.develocity/`

### Are these changes tested?

No (Checking local git output)

### Are there any user-facing changes?

No

Authored-by: Laurent Goujon <laurent@apache.org>
Signed-off-by: David Li <li.davidm96@gmail.com>