docs: fix proxy default timestamp yaml example (#544)

Per the code, the key should use an underscore, not a hyphen. It also seems like the value is parsed as a number instead of a string, which then fails with a type error later, so quote it to force it to be a string.

```
$ pywb
2020-03-10 21:06:33,084: [INFO]: Proxy enabled for collection "web"
Traceback (most recent call last):
  File "/tmp/pywb_venv/bin/pywb", line 8, in <module>
    sys.exit(wayback())
  File "/tmp/pywb_venv/local/lib/python2.7/site-packages/pywb/apps/cli.py", line 20, in wayback
    desc='pywb Wayback Machine Server').run()
  File "/tmp/pywb_venv/local/lib/python2.7/site-packages/pywb/apps/cli.py", line 89, in __init__
    self.application = self.load()
  File "/tmp/pywb_venv/local/lib/python2.7/site-packages/pywb/apps/cli.py", line 181, in load
    return FrontEndApp(custom_config=self.extra_config)
  File "/tmp/pywb_venv/local/lib/python2.7/site-packages/pywb/apps/frontendapp.py", line 79, in __init__
    self.init_proxy(config)
  File "/tmp/pywb_venv/local/lib/python2.7/site-packages/pywb/apps/frontendapp.py", line 569, in init_proxy
    if not self.ALL_DIGITS.match(self.proxy_default_timestamp):
TypeError: expected string or buffer
```

docs/manual/configuring.rst

@@ -388,7 +388,7 @@ The timestamp can also be optionally specified by running: ``wayback --proxy my-
 
   proxy:
     coll: my-coll
-    default-timestamp: 20181226010203
+    default_timestamp: "20181226010203"
 
 The ISO date format, eg. ``2018-12-26T01:02:03`` is also accepted.