Update samples/sample_definition_and_criteria.rst

Co-authored-by: Benjamin Cabé <kartben@gmail.com>

samples/sample_definition_and_criteria.rst

@@ -60,7 +60,7 @@ Sample Criteria
     run on the documented platforms. Use the ``harness`` option to skip the
     execution. Twister only attempts to flash and execute the build binary if
     the harness is set to ``ztest`` or ``console``.
-  * The default configuration for the sample should be in the ``prj.conf``
+  * The default configuration for the sample must be in the :file:`prj.conf`
     file and should be operational on all supported platforms. Do not rely on the
     ``extra_args`` or ``extra_configs`` options in the YAML file to build the
     sample.