docs(Server) Fix colors param (#544)

This PR fixes an incorrect docstring.

For the `color` parameter, the API for libtmux actually expects either `256` or `88`.

Author: TravisDart

src/libtmux/server.py

@@ -98,7 +98,7 @@ class Server(EnvironmentMixin):
     config_file = None
     """Passthrough to ``[-f file]``"""
     colors = None
-    """``-2`` or ``-8``"""
+    """``256`` or ``88``"""
     child_id_attribute = "session_id"
     """Unique child ID used by :class:`~libtmux.common.TmuxRelationalObject`"""
     formatter_prefix = "server_"