Documentation: tracing: fix grammar

4a88d44a ("tracing: Remove mentioning of legacy latency_trace file
from documentation") changed a sentence to refer to only one file
instead of two, but the sentence still uses "they".  Fix it.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>