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 <[email protected]>
Signed-off-by: David Li <[email protected]>

.gitignore

@@ -102,4 +102,4 @@ __debug_bin
 .envrc
 
 # Develocity
-.mvn/.develocity.xml
+.mvn/.develocity/