Quote the "proxy_all" option

The "proxy_all" option from the sample configuration not being quoted was throwing this error when running the server:

    Config error: invalid TOML value, did you mean to use a quoted string? at line 38 column 13

This patch makes sure the program will run against the included example config.toml.

config.toml

@@ -35,7 +35,7 @@ usrdir = true
 proxy = { path = "localhost:1966" }
 # proxy_all is optional
 # It will send all requests to the specified server. It also supports streamming.
-proxy_all = localhost:1967
+proxy_all = "localhost:1967"
 # redirect is optional
 redirect = { "/redirect" = "/", "/newdomain" = "gemini://example.net" }