docs: Fix typos in Documentation/trace/ftrace.rst

Fix the usage of "a/the" and improve the readability.

Signed-off-by: Haocheng Xie <xiehaocheng.cn@gmail.com>
Link: https://lore.kernel.org/r/20210531083905.25763-1-xiehaocheng.cn@gmail.com


[jc: tweaked wording slightly]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>