scripts/spelling.txt: add "an user" pattern and fix typo instances

Fix typos and add the following to the scripts/spelling.txt:

  an user||a user
  an userspace||a userspace

I also added "userspace" to the list since it is a common word in Linux.
I found some instances for "an userfaultfd", but I did not add it to the
list.  I felt it is endless to find words that start with "user" such as
"userland" etc., so must draw a line somewhere.

Link: http://lkml.kernel.org/r/1481573103-11329-4-git-send-email-yamada.masahiro@socionext.com
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/kselftest.txt b/Documentation/kselftest.txt
index d431dc8..5bd5903 100644
--- a/Documentation/kselftest.txt
+++ b/Documentation/kselftest.txt
@@ -59,14 +59,14 @@
 =================
 
 You can use kselftest_install.sh tool installs selftests in default
-location which is tools/testing/selftests/kselftest or an user specified
+location which is tools/testing/selftests/kselftest or a user specified
 location.
 
 To install selftests in default location:
    $ cd tools/testing/selftests
    $ ./kselftest_install.sh
 
-To install selftests in an user specified location:
+To install selftests in a user specified location:
    $ cd tools/testing/selftests
    $ ./kselftest_install.sh install_dir