Fixed mistake in docs (use str.join instead of str.append).

docs/tutorial/multiline.rst

@@ -95,7 +95,7 @@ Which we can verify like so:
 
       @step('I should see the following:')
       def i_should_see_the_following(step):
-          assert step.multiline == '\n'.append([
+          assert step.multiline == '\n'.join([
           ' one',
           '  two  ',
           '   three   ',
@@ -114,4 +114,4 @@ thus doesn't need to have a quote at the end of it.
 
 Also note that if you want a double quote at the beginning of a line
 in your string, you'll have to start your line with two double quotes,
-since the first one will be stripped off.
+since the first one will be stripped off.
\ No newline at end of file