Fix miscellaneous Sphinx warnings (#244)

* /index.rst:24: WARNING: Line block ends without a blank line.

* /install.rst:300: WARNING: Explicit markup ends without a blank line; unexpected unindent.

* /admin.rst:273: WARNING: Could not lex literal_block as "http". Highlighting skipped.

* /tutorials/tut1.rst:150: WARNING: Could not lex literal_block as "json". Highlighting skipped.

* Remove extra newline

* Use integer value as epoch in JSON snippet

Even though using a string value is convenient (due to the lack of JSON
comments), it is confusing since the value should actually be an
integer. A slightly more verbose documentation is therefore preferable.

tutorials/tut1.rst

@@ -151,9 +151,11 @@ Go back to jwt.io and change the payload to
 
   {
     "role": "todo_user",
-    "exp": <computed epoch value>
+    "exp": 123456789
   }
 
+**NOTE**: Don't forget to change the dummy epoch value :code:`123456789` in the snippet above to the epoch value returned by the psql command.
+
 Copy the updated token as before, and save it as a new environment variable.
 
 .. code-block:: bash