Update UPGRADE-5.1.md

I guess there was a mistake on the Config upgrade. The method's name which signature has changed is `NodeDefinition::setDeprecated(...)` (instead of `NodeDefinition::setDeprecation(...)`).
Same thing goes for `BaseNode::setDeprecated(...).

Thanks :)

UPGRADE-5.1.md

@@ -4,8 +4,8 @@ UPGRADE FROM 5.0 to 5.1
 Config
 ------
 
- * The signature of method `NodeDefinition::setDeprecated()` has been updated to `NodeDefinition::setDeprecation(string $package, string $version, string $message)`.
- * The signature of method `BaseNode::setDeprecated()` has been updated to `BaseNode::setDeprecation(string $package, string $version, string $message)`.
+ * The signature of method `NodeDefinition::setDeprecated()` has been updated to `NodeDefinition::setDeprecated(string $package, string $version, string $message)`.
+ * The signature of method `BaseNode::setDeprecated()` has been updated to `BaseNode::setDeprecated(string $package, string $version, string $message)`.
  * Passing a null message to `BaseNode::setDeprecated()` to un-deprecate a node is deprecated
  * Deprecated `BaseNode::getDeprecationMessage()`, use `BaseNode::getDeprecation()` instead