config: fix 'instance not found' failure message

The instance config schema was modified in commit 8ee2b0d8
("config: start singleton instance in RW by default"): the `database.rw`
(boolean) parameter was replaced by the `database.mode` parameter (enum
of `ro` and `rw`).

However, in the same commit, the default mode of an instance that is the
only one in its replicaset was modified: it starts in the read-write
mode by default. As result, the parameter is actually unneeded in the
minimal configuration example.

These changes were not reflected in the 'instance not found' error
message, which contains the minimal configuration example. It is fixed
here.

Part of #8862
Follows up #8810

NO_DOC=nothing to document, it is just an error message
NO_CHANGELOG=see NO_DOC
NO_TEST=there is a test case in config-luatest/reload_test.lua that
        triggers the given error; it doesn't needs updating, because it
        checks the start of the error message