trace: fix minor typo
trace: fix minor typo

Fixes #722.

Change-Id: I5abcbac84d6642d8bda3a93a31cec67f7e4730f7
Reviewed-on: https://code-review.googlesource.com/15870
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go