MINOR: [DEV] Add sgilmore10 to CODEOWNERS for /matlab/ (apache#42125)

### Rationale for this change

Adding myself as a CODEOWNER for `/matlab/` so I can help review MATLAB PRs.

Authored-by: Sarah Gilmore <sgilmore@mathworks.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>

.github/CODEOWNERS

@@ -37,7 +37,7 @@
 /go/ @zeroshade
 /java/ @lidavidm
 /js/ @domoritz @trxcllnt
-/matlab/ @kevingurney @kou
+/matlab/ @kevingurney @kou @sgilmore10
 /python/pyarrow/_flight.pyx @lidavidm
 /python/pyarrow/**/*gandiva* @wjones127
 /r/ @paleolimbot @thisisnic