Make limit example a working example.

The current `./run-jerikan build
--limit=gateway\?.ussfo03.blade-group.net,none` results in a fairly
complicated error message about BGPttH:

```
self = <jerikan.jinja.ErrorExtension object at 0x7fb938300a90>
message = "no BGPttH information (don't generate gateway templates
alone)"
caller = <Macro anonymous>

    def _raise(self, message, caller):
        """Execute the {% error %} statement, raising an exception."""
>       raise TemplateRuntimeError(message)
E       jinja2.exceptions.TemplateRuntimeError: no BGPttH information
(don't generate gateway templates alone)

caller     = <Macro anonymous>
message    = "no BGPttH information (don't generate gateway templates
alone)"
self       = <jerikan.jinja.ErrorExtension object at 0x7fb938300a90>

jerikan/jinja.py:365: TemplateRuntimeError
------------------------------------ Captured log call
------------------------------------
INFO     jerikan.build:build.py:112 build template data.j2 to data.yaml
for gateway2.ussfo03.blade-group.net
INFO     jerikan.build:build.py:112 build template linux/interfaces.j2
to network-interfaces for gateway2.ussfo03.blade-group.net
INFO     jerikan.build:build.py:112 build template linux/keepalived.j2
to keepalived.conf for gateway2.ussfo03.blade-group.net
INFO     jerikan.build:build.py:213 skip empty template
gateway2.ussfo03.blade-group.net for linux/keepalived.j2
INFO     jerikan.build:build.py:112 build template
linux/authorized-keys.j2 to authorized_keys for
gateway2.ussfo03.blade-group.net
INFO     jerikan.build:build.py:112 build template
linux/nftables-rules-v4.j2 to nftables.conf for
gateway2.ussfo03.blade-group.net
INFO     jerikan.build:build.py:112 build template linux/sysctl.conf.j2
to sysctl.conf for gateway2.ussfo03.blade-group.net
INFO     jerikan.build:build.py:112 build template linux/motd.j2 to motd
for gateway2.ussfo03.blade-group.net
INFO     jerikan.build:build.py:112 build template linux/dhcp.j2 to
dhcpd.conf for gateway2.ussfo03.blade-group.net
INFO     jerikan.build:build.py:112 build template linux/nginx.j2 to
nginx.conf for gateway2.ussfo03.blade-group.net
-------------------- generated xml file: /app/jerikan/output/junit.xml
--------------------
--------------- generated html file:
file:///app/jerikan/output/report.html ---------------
================================= short test summary info
=================================
FAILED jerikan/build.py::build[gateway1.ussfo03.blade-group.net] -
jinja2.exceptions.Tem...
FAILED jerikan/build.py::build[gateway2.ussfo03.blade-group.net] -
jinja2.exceptions.Tem...
```

README.md

@@ -31,7 +31,7 @@ $ ./run-jerikan build
 To limit to a few devices:
 
 ```console
-$ ./run-jerikan build --limit=gateway\?.ussfo03.blade-group.net,none
+$ ./run-jerikan build --limit=spine\?.ussfo03.blade-group.net,none
 ```
 
 ## Get the scope of a device