doc: dev-tools: kselftest.rst: update config file location

Config fragment files should be placed in
tools/testing/selftests/<testdir>/config

Signed-off-by: Anders Roxell <[e-mail]>
Acked-by: Shuah Khan (Samsung OSG) <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/dev-tools/kselftest.rst

···
  * If a test needs specific kernel config options enabled, add a config file in
    the test directory to enable them.
 
-   e.g: tools/testing/selftests/android/ion/config
+   e.g: tools/testing/selftests/android/config
 
 Test Harness
 ============