docs: ftrace: Fix typos

Fix minor typos in the doc.

Suggested-by: Randy Dunlap <[e-mail]>
Reviewed-by: Steven Rostedt (VMware) <[e-mail]>
Reviewed-by: Joel Fernandes (Google) <[e-mail]>
Signed-off-by: Frank A. Cancio Bello <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/trace/ftrace.rst

···
 	This interface also allows for commands to be used. See the
 	"Filter commands" section for more details.
 
-	As a speed up, since processing strings can't be quite expensive
+	As a speed up, since processing strings can be quite expensive
 	and requires a check of all functions registered to tracing, instead
 	an index can be written into this file. A number (starting with "1")
 	written will instead select the same corresponding at the line position
···
 
 	By default, 128 comms are saved (see "saved_cmdlines" above). To
 	increase or decrease the amount of comms that are cached, echo
-	in a the number of comms to cache, into this file.
+	the number of comms to cache into this file.
 
   saved_tgids:
 
···
 
 As you can see, the new directory looks similar to the tracing directory
 itself. In fact, it is very similar, except that the buffer and
-events are agnostic from the main director, or from any other
+events are agnostic from the main directory, or from any other
 instances that are created.
 
 The files in the new directory work just like the files with the