fix import numpy in README.rst

README.rst

@@ -33,7 +33,7 @@ Mars tensor provides a familiar interface like Numpy.
 +------------------------------------------------+----------------------------------------------------+
 |.. code-block:: python                          |.. code-block:: python                              |
 |                                                |                                                    |
-|    import numpy                                |    import mars.tensor as mt                        |
+|    import numpy as np                          |    import mars.tensor as mt                        |
 |    a = np.random.rand(1000, 2000)              |    a = mt.random.rand(1000, 2000)                  |
 |    (a + 1).sum(axis=1)                         |    (a + 1).sum(axis=1).execute()                   |
 |                                                |                                                    |
@@ -183,4 +183,4 @@ Thank you in advance for your contributions!
 .. |Implementation| image:: https://img.shields.io/pypi/implementation/pymars.svg?style=flat-square
 .. _`mars-dev@googlegroups.com`: https://groups.google.com/forum/#!forum/mars-dev
 .. _`GitHub issue`: https://github.com/mars-project/mars/issues
-.. _`pull requests`: https://github.com/mars-project/mars/pulls
+.. _`pull requests`: https://github.com/mars-project/mars/pulls