Update README for run-tempest role

Updates README in order to clear confusion about defining
multi-line regexes.

Closes-Bug: 1827234
Change-Id: Ie55ef879b1d6c971b202553b3906d02c29b981d6
diff --git a/roles/run-tempest/README.rst b/roles/run-tempest/README.rst
index 1f7fb70..91b0b5f 100644
--- a/roles/run-tempest/README.rst
+++ b/roles/run-tempest/README.rst
@@ -20,14 +20,12 @@
    It works only when used with some specific tox environments
    ('all', 'all-plugin'.)
 
-   Multi-line and commented regexs can be achieved by doing this:
+   In the following example only api scenario and third party tests
+   will be executed.
 
        ::
            vars:
-             tempest_test_regex: |
-               (?x)    # Ignore comments and whitespaces
-               # Line with only a comment.
-               (tempest\.(api|scenario|thirdparty)).*$    # Run only api scenario and third party
+             tempest_test_regex: (tempest\.(api|scenario|thirdparty)).*$
 
 .. zuul:rolevar:: tempest_test_blacklist
 
@@ -48,14 +46,9 @@
    It works only when used with some specific tox environments
    ('all', 'all-plugin'.)
 
-   Multi-line and commented regexs can be achieved by doing this:
-
        ::
            vars:
-             tempest_black_regex: |
-               (?x)    # Ignore comments and whitespaces
-               # Line with only a comment.
-               (tempest.api.identity).*$
+             tempest_black_regex: (tempest.api.identity).*$
 
 .. zuul:rolevar:: tox_extra_args
    :default: ''