tracing/doc: Fix typos in histogram-design.rst

There's a few typos in the histogram-design.rst document that need need to
be fixed.

Cc: Tom Zanussi <zanussi@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/histogram-design.rst

@@ -700,7 +700,7 @@ variable, as well as the referenced variable's size, type, and
 is_signed values.  The VAR_REF field's .name is set to the name of the
 variable it references.  If a variable reference was created using the
 explicit system.event.$var_ref notation, the hist_field's system and
-event_name variabls are also set.
+event_name variables are also set.
 
 So, in order to handle an event for the sched_switch histogram,
 because we have a reference to a variable on another histogram, we
@@ -1445,7 +1445,7 @@ reference to the variable being tracked, in this case the $wakeup_lat
 variable.  In order to perform the onmax() handler function, there
 also needs to be a variable that tracks the current maximum by getting
 updated whenever a new maximum is hit.  In this case, we can see that
-an autogenerated veriable named ' __max' has been created and is
+an auto-generated variable named ' __max' has been created and is
 visible in the actions[].track_data.track_var variable.
 
 Finally, in the new 'save action variables' section, we can see that
@@ -1611,7 +1611,7 @@ A couple special cases
 
 While the above covers the basics of the histogram internals, there
 are a couple of special cases that should be discussed, since they
-tend to creae even more confusion.  Those are field variables on other
+tend to create even more confusion.  Those are field variables on other
 histograms, and aliases, both described below through example tests
 using the hist_debug files.