spelling: fix grammar in cinder_test_runner312.py comment

Summary:
Fix a missing relative pronoun in the file header comment. The
sentence "This compensates for JIT compilation, particularly for
debug builds, is a huge overhead." was missing "which" after
"compilation", making it grammatically incorrect.

Reviewed By: yoney

Differential Revision: D99701389

fbshipit-source-id: ad65909718555f149023528a521637159a37ad3f

Author: alexmalyshev

cinderx/TestScripts/cinder_test_runner312.py

@@ -1,7 +1,7 @@
 # Copyright (c) Meta Platforms, Inc. and affiliates.
 #
 # A modified version of the regrtest system which runs multiple tests per
-# instance of a parallel worker. This compensates for JIT compilation,
+# instance of a parallel worker. This compensates for JIT compilation which,
 # particularly for debug builds, is a huge overhead. So, we amortize initial
 # startup costs.
 #