[ Service ] Update IsolateGroup service documentation
[ Service ] Update IsolateGroup service documentation

Fixed instances where we were using 'isolate' instead of 'isolate
group'.

Fixes #46374

TEST=N/A

Change-Id: I6820954b6cbcce0c5205d46dba334691c426147c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/205321
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk