docs: resolving namespace conflict, refactored variable 
In docs of about_arrow.md, in the below example code
![image](https://github.com/huggingface/datasets/assets/74114936/fc70e152-e15f-422e-949a-1c4c4c9aa116)
The variable name 'time' was being used in a way that could potentially lead to a namespace conflict with Python's built-in 'time' module. It is not a good convention and can lead to unintended variable shadowing for any user re-using the example code.
To ensure code clarity, and prevent potential naming conflicts renamed the variable 'time' to 'elapsed_time' in the example  code.