bluetooth: gd: update gd/cert/run instruction

Mobly test runner uses "." instead of ":" to separate test from test
class, update --help instruction to reflect that.

BUG: 240781725
Test: gd/cert/run L2capTest.test_receiving_i_frames_with_fcs_for_ertm
Change-Id: I15b8a7db32b47b3d8482b5e5db31c866acee7ff7