fix: rename cargo config for xtask alias

[chenyukang]

cargo will read configuration .cargo/config.toml by default: https://doc.rust-lang.org/cargo/reference/config.html?highlight=alias#hierarchical-structure

otherwise the instruction here does not work because cargo can not find alias:

ractor/xtask/README.md

Line 12 in 4683adb

cargo xtask coverage --dev

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.39%. Comparing base (9ebeece) to head (4683adb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #338   +/-   ##
=======================================
  Coverage   82.39%   82.39%           
=======================================
  Files          65       65           
  Lines       12917    12917           
=======================================
  Hits        10643    10643           
  Misses       2274     2274           

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