MINOR: [Dev] Update CODEOWNERS for kou (#36572)

### Rationale for this change

I'm reviewing MATLAB/Swift related PRs because there are not enough committers for them for now.

### What changes are included in this PR?

Add me to MATLAB/Swift.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>

.github/CODEOWNERS

@@ -42,12 +42,13 @@
 /go/ @zeroshade
 /java/ @lidavidm
 /js/ @domoritz @trxcllnt
-/matlab/ @kevingurney
+/matlab/ @kevingurney @kou
 /python/ @AlenkaF
 /python/pyarrow/_flight.pyx @lidavidm
 /python/pyarrow/**/*gandiva* @wjones127
 /r/ @paleolimbot @thisisnic
 /ruby/ @kou
+/swift/ @kou
 
 # Docs
 # /docs/