Documentation: kselftest: fix codeblock

Add extra colon to mark command in the next paragraph as codeblock

Signed-off-by: Yo-Jung (Leo) Lin <0xff07@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kselftest.rst

@@ -183,7 +183,7 @@ expected time it takes to run a test. If you have control over the systems
 which will run the tests you can configure a test runner on those systems to
 use a greater or lower timeout on the command line as with the `-o` or
 the `--override-timeout` argument. For example to use 165 seconds instead
-one would use:
+one would use::
 
    $ ./run_kselftest.sh --override-timeout 165