doc: avoid double-while sentence in perf_hooks.md

This improves readability, as well as awkward reptition of the word
_while_ with two different meanings in a single sentence.

Refs: https://docs.microsoft.com/en-us/style-guide/a-z-word-list-term-collections/w/while

PR-URL: nodejs#35078
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>

Author: Trott

doc/api/perf_hooks.md

@@ -101,7 +101,7 @@ setImmediate(() => {
 });
 ```
 
-While the CPU is mostly idle while running this script the value of
+Although the CPU is mostly idle while running this script, the value of
 `utilization` is 1. This is because the call to [`child_process.spawnSync()`][]
 blocks the event loop from proceeding.