tutorial: Update type of BaseUrl to real type

I got confused to why there was one more argument passed to
constructor than the type takes. Turns out the docs were just a bit out
of date.

doc/tutorial/Client.lhs

@@ -107,6 +107,7 @@ data BaseUrl = BaseUrl
   { baseUrlScheme :: Scheme -- ^ URI scheme to use
   , baseUrlHost :: String   -- ^ host (eg "haskell.org")
   , baseUrlPort :: Int      -- ^ port (eg 80)
+  , baseUrlPath :: String   -- ^ path (eg "/a/b/c")
   }
 ```