[PWGCF] fix bug for the resolution calculation

[jikim1290]

fix bug for the resolution calculation

[github-actions]

O2 linter results: ❌ 3 errors, ⚠️ 5 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/staging for 33a85d2 at 2026-06-02 01:51:

## sw/BUILD/O2Physics-latest/log
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/PWGCF/Flow/Tasks/flowDirectedFlowTask.cxx:525:11: error: unused variable 'resDot' [-Werror=unused-variable]
ninja: build stopped: subcommand failed.

Full log here.

[alibuild]

Error while checking build/O2Physics/o2 for 33a85d2 at 2026-06-02 01:52:

## sw/BUILD/O2Physics-latest/log
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/PWGCF/Flow/Tasks/flowDirectedFlowTask.cxx:525:11: error: unused variable 'resDot' [-Werror=unused-variable]
ninja: build stopped: subcommand failed.

Full log here.

[alibuild]

Error while checking build/O2Physics/staging for b785cec at 2026-06-02 02:01:

## sw/BUILD/O2Physics-latest/log
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/PWGCF/Flow/Tasks/flowDirectedFlowTask.cxx:520:11: error: unused variable 'magA' [-Werror=unused-variable]
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/PWGCF/Flow/Tasks/flowDirectedFlowTask.cxx:521:11: error: unused variable 'magC' [-Werror=unused-variable]
ninja: build stopped: subcommand failed.

Full log here.

[alibuild]

Error while checking build/O2Physics/o2 for b785cec at 2026-06-02 02:02:

## sw/BUILD/O2Physics-latest/log
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/PWGCF/Flow/Tasks/flowDirectedFlowTask.cxx:520:11: error: unused variable 'magA' [-Werror=unused-variable]
/sw/SOURCES/O2Physics/slc9_x86-64-slc9_x86-64/0/PWGCF/Flow/Tasks/flowDirectedFlowTask.cxx:521:11: error: unused variable 'magC' [-Werror=unused-variable]
ninja: build stopped: subcommand failed.

Full log here.