Fix label_tensor.py doc (mathLab#543)

Author: dario-coscia

pina/label_tensor.py

@@ -58,7 +58,7 @@ def __init__(self, x, labels):
             >>> from pina import LabelTensor
             >>> tensor = LabelTensor(
             >>>     torch.rand((2000, 3)),
-            ...     {1: {"name": "space", "dof": ['a', 'b', 'c'])
+            ...     {1: {"name": "space", "dof": ['a', 'b', 'c']}})
             >>> tensor = LabelTensor(
             >>>     torch.rand((2000, 3)),
             ...     ["a", "b", "c"])