docs: ftrace: Fix typos

Fix minor typos in the doc.
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
Link: https://lore.kernel.org/r/9ef705d0208a4ca0852fed69bc0838a589a4df85.1577231751.git.frank@generalsoftwareinc.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/ftrace.rst

@@ -238,7 +238,7 @@ of ftrace. Here is a list of some of the key files:
 	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
@@ -385,7 +385,7 @@ of ftrace. Here is a list of some of the key files:
 
 	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:
 
@@ -3330,7 +3330,7 @@ directories after it is created.
 
 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