Attempting to fix docstyle

Author: ngayraud

ot/utils.py

@@ -189,9 +189,9 @@ def cost_normalization(C, norm=None):
     elif norm == "loglog":
         C = np.log1p(np.log1p(C))
     else:
-        raise ValueError(f'Norm {norm} is not a valid option.\n'
-                         f'Valid options are:\n'
-                         f'median, max, log, loglog')
+        raise ValueError('Norm %s is not a valid option.\n'
+                         'Valid options are:\n'
+                         'median, max, log, loglog' % norm)
     return C