minor #21632 Update configuration formats mentioned in docs (vasilvestre)

This PR was merged into the 8.0 branch.

Discussion
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 Update configuration formats mentioned in docs

Since XML is abandonned, there's only two formats now.

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Commits
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e4874b9 Update configuration formats mentioned in docs

Author: javiereguiluz

configuration.rst

@@ -55,7 +55,7 @@ Configuration Formats
 Unlike other frameworks, Symfony doesn't impose a specific format on you to
 configure your applications, but lets you choose between YAML and PHP.
 Throughout the Symfony documentation, all configuration examples will be
-shown in these three formats.
+shown in these two formats.
 
 There isn't any practical difference between formats. In fact, Symfony
 transforms all of them into PHP and caches them before running the application,