Updates Source Interface docs language

Makes two updates to the text of the Source Interface in order to match
the new screenshots:

Renames Source Interface button "Logout" -> "Exit" in docs

The name of the "logout" button as presented in the Source Interface
changed back in #1534, but the corresponding documentation was never
updated. Now that the screenshots show the new "Exit" button, we must
also update the guiding language to refer explicitly to an "Exit"
button, not a "Logout" button.

Updates Source codename displayed in-line to match the screenshots.
Since Source codenames are randomly generated, updating the screenshots
means we'll definitely get a new codename, so let's update the in-line
text to match what's displayed in the visual aids, to avoid confusion.

docs/source.rst

@@ -79,7 +79,7 @@ time you are using SecureDrop, click the **Submit Documents** button.
 
 You should now see a screen that shows the unique codename that SecureDrop has
 generated for you. In the example screenshot below the codename is
-``bagpipe aloof whiff borrowing overwrite datebook unadvised``, but yours will
+``catering zit isotope consonant tiny shriek provider``, but yours will
 be different. It is extremely important that you both remember this code and
 keep it secret. After submitting documents, you will need to provide this code
 to log back in and check for responses.
@@ -118,7 +118,7 @@ messages.
 |Confirmation page|
 
 Once you are finished submitting documents, be certain you have saved your
-secret codename and then click the **Logout** button:
+secret codename and then click the **Exit** button:
 
 |Logout|