[Sync EN] Fix SplFixedArray example for non-integer key error type (#5227)

[github-actions]

Commit: https://github.com/php/doc-en/commit/a833060594749e44ff296fee01fd587d6395cff7
PR: https://github.com/php/doc-en/pull/5227

Archivos ES a actualizar

• reference/spl/splfixedarray.xml