selftests/Makefile: Add missed closing `"` in comment

A comment for make command usage in `selftets/Makefile` has opening `"`
but no closing `"`.  This commit adds the missed `"` in the comment.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/Makefile

@@ -39,7 +39,7 @@ TARGETS += x86
 TARGETS += zram
 #Please keep the TARGETS list alphabetically sorted
 # Run "make quicktest=1 run_tests" or
-# "make quicktest=1 kselftest from top level Makefile
+# "make quicktest=1 kselftest" from top level Makefile
 
 TARGETS_HOTPLUG = cpu-hotplug
 TARGETS_HOTPLUG += memory-hotplug