[DOC] a few documentations for linalg ops are broken

[eric-haibin-lin]

http://mxnet.incubator.apache.org/api/python/ndarray/linalg.html?highlight=lin#mxnet.ndarray.linalg.gemm
and
http://mxnet.incubator.apache.org/api/python/ndarray/linalg.html?highlight=lin#mxnet.ndarray.linalg.gemm2

source code in https://github.com/apache/incubator-mxnet/blob/master/src/operator/tensor/la_op.cc#L54-L61

[mxnet-label-bot]

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[frankfliu]

@mxnet-label-bot add [Doc]