Fix per-file rule in pkg/frontend_server/OWNERS

Extraneous spaces are not allowed in per-file rules

Change-Id: I1249661dcd76b836d27b2a3be0a41c25e3bc0b6f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395480
Reviewed-by: Johnni Winther <[email protected]>

Author: derekxu16

pkg/frontend_server/OWNERS

@@ -4,4 +4,4 @@ file:/tools/OWNERS_CFE
 file:/OWNERS
 
 # Add the VM team as owners of ResidentFrontendServer-related files.
-per-file lib/src/resident_frontend_server.dart, test/src/resident_frontend_server_test.dart = file:/tools/OWNERS_VM
+per-file lib/src/resident_frontend_server.dart,test/src/resident_frontend_server_test.dart=file:/tools/OWNERS_VM