Fix remote_topgrade_path example value (#486)

The `remote_topgrade_path` expects a string, not an array.

config.example.toml

@@ -23,7 +23,7 @@
 #ssh_arguments = "-o ConnectTimeout=2"
 
 # Path to Topgrade executable on remote machines
-#remote_topgrade_path = [".cargo/bin/topgrade"]
+#remote_topgrade_path = ".cargo/bin/topgrade"
 
 # Arguments to pass tmux when pulling Repositories
 #tmux_arguments = "-S /var/tmux.sock"