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>

Documentation/trace/ftrace.rst

@@ -354,8 +354,8 @@ of ftrace. Here is a list of some of the key files:
 	is being directly called by the function. If the count is greater
 	than 1 it most likely will be ftrace_ops_list_func().
 
-	If the callback of the function jumps to a trampoline that is
-	specific to a the callback and not the standard trampoline,
+	If the callback of a function jumps to a trampoline that is
+	specific to the callback and which is not the standard trampoline,
 	its address will be printed as well as the function that the
 	trampoline calls.