minor #54481 [DependencyInjection] Document BC break in UPGRADE file (OskarStark)

This PR was squashed before being merged into the 7.1 branch.

Discussion
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[DependencyInjection] Document BC break in UPGRADE file

| Q             | A
| ------------- | ---
| Branch?       | 7.1
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | Follows #52843
| License       | MIT

Commits
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055892d7d3 [DependencyInjection] Document BC break in UPGRADE file

Author: fabpot

CHANGELOG.md

@@ -11,7 +11,7 @@ CHANGELOG
  * Add `#[AutowireMethodOf]` attribute to autowire a method of a service as a callable
  * Make `ContainerBuilder::registerAttributeForAutoconfiguration()` propagate to attribute classes that extend the registered class
  * Add argument `$prepend` to `FileLoader::construct()` to prepend loaded configuration instead of appending it
- * [BC BREAK] When used in the `prependExtension()` methods, the `ContainerConfigurator::import()` method now prepends the configuration instead of appending it
+ * [BC BREAK] When used in the `prependExtension()` method, the `ContainerConfigurator::import()` method now prepends the configuration instead of appending it
 
 7.0
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