!13057 fix the format error of BatchMatMul operator.

From: @wangshuide2020
Reviewed-by: @liangchenghui,@wuxuejian
Signed-off-by: @liangchenghui

mindspore/ops/operations/math_ops.py

@@ -800,7 +800,7 @@ class MatMul(PrimitiveWithCheck):
 
 
 class BatchMatMul(MatMul):
-    """
+    r"""
     Computes matrix multiplication between two tensors by batch.
 
     .. math::