Style sigmoid function in harmony with pep guideness (TheAlgorithms#6677)

* Style sigmoid function in harmony with pep guideness

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Apply suggestions from code review

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Tianyi Zheng <tianyizheng02@gmail.com>

Author: 3 people

neural_network/back_propagation_neural_network.py

@@ -21,8 +21,8 @@
 from matplotlib import pyplot as plt
 
 
-def sigmoid(x):
-    return 1 / (1 + np.exp(-1 * x))
+def sigmoid(x: np.ndarray) -> np.ndarray:
+    return 1 / (1 + np.exp(-x))
 
 
 class DenseLayer: