DnsResolver/res_send.cpp - fixes bugprone-argument-comment warning

Fixes:
  packages/modules/DnsResolver/res_send.cpp:868:38: error: argument name 'sigmask' in comment does not match parameter name '__mask' [bugprone-argument-comment,-warnings-as-errors]
      int n = ppoll(&fds, 1, &timeout, /*sigmask=*/NULL);
                                       ^~~~~~~~~~~~
                                       /*__mask=*/
  bionic/libc/include/poll.h:64:129: note: '__mask' declared here
  int ppoll(struct pollfd* _Nullable __fds, nfds_t __count, const struct timespec* _Nullable __timeout, const sigset_t* _Nullable __mask) __INTRODUCED_IN(21);
                                                                                                                                  ^
  packages/modules/DnsResolver/res_send.cpp:907:67: error: argument name 'sigmask' in comment does not match parameter name '__mask' [bugprone-argument-comment,-warnings-as-errors]
          const int n = ppoll(fdset.data(), fdset.size(), &timeout, /*sigmask=*/nullptr);
                                                                    ^~~~~~~~~~~~
                                                                    /*__mask=*/
  bionic/libc/include/poll.h:64:129: note: '__mask' declared here
  int ppoll(struct pollfd* _Nullable __fds, nfds_t __count, const struct timespec* _Nullable __timeout, const sigset_t* _Nullable __mask) __INTRODUCED_IN(21);

Test: builds
Bug: 153035880
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Iff535accfc09968f070a93daff9e4a5aded2e4bc