Doc: README uses YAML config in Example Migration but only mentions JSON format in Configuration

Synapse can use both YAML and JSON format configuration files, and in Example Migration section of README.md, the example Synapse configuration is in YAML format. However Configuration section states "Synapse depends on a single config file in JSON format; it's usually called synapse.conf.json". This is not consistent with Synapse capability or the example config. Please fix by either change the example config file to JSON, or preferably, state that both JSON and YAML are valid for configuration.

Open README.md and compare the Configuration section with the YAML example in the Example Migration section. Update the Configuration wording to state that both JSON and YAML configuration files are valid, then verify that the README no longer contradicts the example.

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