fix: existsMemberByEmail에 RequestBody 추가

Author: rjswjddn

src/main/java/com/oronaminc/join/member/api/MemberController.java

@@ -46,7 +46,7 @@ public class MemberController {
     @GetMapping("/exists")
     @ResponseStatus(HttpStatus.OK)
     public ExistsMemberResponse existsMemberByEmail(
-        @Valid ExistsMemberRequest existsMemberRequest) {
+        @Valid @RequestBody ExistsMemberRequest existsMemberRequest) {
         boolean exists = memberService.existsMemberByEmail(existsMemberRequest.email());
         return new ExistsMemberResponse(exists);
     }