BUG: Remove spare f in an f-string

Remove a spare `f` when redirecting the output of a 3D Slicer command
call using an f-string in the transform hardening script.

Inadvertently introduced in commit ac6040f.

bin/wm_harden_transform.py

@@ -115,7 +115,7 @@ def command_harden_transform(polydata, transform, inverse, slicer_path, outdir):
             str_inverse = 0
 
         print(f"<{os.path.basename(__file__)}> Transforming: {polydata}")
-        cmd = f"{slicer_path} --no-main-window --python-script $(which harden_transform_with_slicer.py) {polydata} {transform} {str(str_inverse)} {outdir} --python-code 'slicer.app.quit()' >> f{os.path.join(outdir, 'log.txt 2>&1')}"
+        cmd = f"{slicer_path} --no-main-window --python-script $(which harden_transform_with_slicer.py) {polydata} {transform} {str(str_inverse)} {outdir} --python-code 'slicer.app.quit()' >> {os.path.join(outdir, 'log.txt 2>&1')}"
 
         os.system(cmd)