fix sample code error of paddle.lerp, test=document_fix (#42753)

修复paddle.lerp中示例代码错误。

python/paddle/tensor/math.py

@@ -3561,7 +3561,7 @@ def lerp(x, y, weight, name=None):
             x = paddle.arange(1., 5., dtype='float32')
             y = paddle.empty([4], dtype='float32')
             y.fill_(10.)
-            out = paddle.lerp(start, end, 0.5)
+            out = paddle.lerp(x, y, 0.5)
             # out: [5.5., 6., 6.5, 7.]
 
     """