[Sync EN] Fix grammar in predefined (#5755)

[github-actions]

Commit: https://github.com/php/doc-en/commit/029f19dbc090e4d249a23c0ab087d47059b37adc
PR: https://github.com/php/doc-en/pull/5755

Zu aktualisierende DE-Dateien

• language/predefined/arrayaccess.xml
• language/predefined/iterator.xml
• language/predefined/serializable.xml
• language/predefined/stdclass.xml
• language/predefined/traversable.xml
• language/predefined/valueerror.xml
• language/predefined/weakmap.xml
• language/predefined/weakreference.xml