!12285 fix bugs of op BCELoss in API

From: @lihongkang1
Reviewed-by: @liangchenghui,@wuxuejian
Signed-off-by: @liangchenghui

mindspore/nn/loss/loss.py

@@ -606,9 +606,9 @@ class BCELoss(_Loss):
 
     .. math::
         \ell(x, y) = \begin{cases}
-        L, & \text{if reduction} = \text{`none';}\\
-        \operatorname{mean}(L), & \text{if reduction} = \text{`mean';}\\
-        \operatorname{sum}(L),  & \text{if reduction} = \text{`sum'.}
+        L, & \text{if reduction} = \text{'none';}\\
+        \operatorname{mean}(L), & \text{if reduction} = \text{'mean';}\\
+        \operatorname{sum}(L),  & \text{if reduction} = \text{'sum'.}
         \end{cases}
 
     Note: