!12681 Fix error format of api comments.

From: @zhang_yi2020
Reviewed-by: @liangchenghui,@zhunaipan
Signed-off-by: @liangchenghui

mindspore/nn/optim/momentum.py

@@ -49,13 +49,13 @@ class Momentum(Optimizer):
 
     If use_nesterov is True:
 
-        .. math::
-                p_{t} =  p_{t-1} - (grad \ast lr + v_{t} \ast u \ast lr)
+    .. math::
+            p_{t} =  p_{t-1} - (grad \ast lr + v_{t} \ast u \ast lr)
 
     If use_nesterov is Flase:
 
-        .. math::
-                p_{t} = p_{t-1} - lr \ast v_{t}
+    .. math::
+            p_{t} = p_{t-1} - lr \ast v_{t}
 
     Here: where grad, lr, p, v and u denote the gradients, learning_rate, params, moments, and momentum respectively.