fix error words for api comments.

mindspore/nn/optim/momentum.py

@@ -52,7 +52,7 @@ class Momentum(Optimizer):
     .. math::
             p_{t} =  p_{t-1} - (grad \ast lr + v_{t} \ast u \ast lr)
 
-    If use_nesterov is Flase:
+    If use_nesterov is False:
 
     .. math::
             p_{t} = p_{t-1} - lr \ast v_{t}

mindspore/nn/optim/sgd.py

@@ -47,7 +47,7 @@ class SGD(Optimizer):
     .. math::
             p_{t+1} = p_{t} - lr \ast (gradient + u \ast v_{t+1})
 
-    If nesterov is Flase:
+    If nesterov is False:
 
     .. math::
             p_{t+1} = p_{t} - lr \ast v_{t+1}