Updated --output command description in README

[Kalmalyzer]

Current versions of jenkins-plugin-cli support --output switch values: stdout, yaml, txt. (Reference:

plugin-installation-manager-tool/plugin-management-library/src/main/java/io/jenkins/tools/pluginmanager/config/OutputFormat.java

Lines 3 to 5 in 28b405f

	public enum OutputFormat {
	STDOUT, YAML, TXT
	}

)

The command-line documentation is up-to-date as the values are autogenerated via reflection. However, the README contains stale information.

This change brings the README in line with the application itself, as far as the --output switch is concerned.

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