STYLE: PrintSlice re-used variable name

[hjmjohnson]

The variable name s was used as an input and a loop variable. Split the names to clearly indicate the iteration variable intent.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)

[hjmjohnson]

/azp
ITK.macOS

[hjmjohnson]

/azp ITK.macOS

[hjmjohnson]

/azp ITK.macOS

[hjmjohnson]

Failing test is not related:

107:  Test 009: Specialised version is faster                         --> **FAILED**
107: max_dsq: 4.77266e-27  mean_dsq: 2.40117e-29
107:  Test 010: Specialised version gives similar results             -->   PASSED
107:  Test 011: Numerically difficult values are ok v1                -->  should be 4199, is 4199, difference 1.81899e-12,   PASSED
107:  Test 012: Numerically difficult values are ok v2                -->  should be 4199, is 4199, difference 1.81899e-12,   PASSED
107:  Test 013: Numerically difficult values are ok v3                -->  should be 4801, is 4801, difference 4.54747e-12,   PASSED
107: -----------------------------------------------------------------------------
107: test_symmetric_eigensystem Test Summary: 12 tests succeeded, 1 test failed		*****
107: -----------------------------------------------------------------------------
 122/3246 Test  #107: vnl_algo_test_symmetric_eigensystem ....................................................................***Failed    0.20 sec

Addressed upstream in VXL vxl/vxl@6f2a5eb

vxl/vxl#970