Update CONTRIBUTING.md (TheAlgorithms#1886)

Author: SandersLin

CONTRIBUTING.md

@@ -138,7 +138,7 @@ We want your work to be readable by others; therefore, we encourage you to note
   The use of [Python type hints](https://docs.python.org/3/library/typing.html) is encouraged for function parameters and return values.  Our automated testing will run [mypy](http://mypy-lang.org) so run that locally before making your submission.
 
   ```python
-  def sum_ab(a: int, b: int) --> int:
+  def sum_ab(a: int, b: int) -> int:
       return a + b
   ```