Signum -> Sign function

Author: mmatera

mathics/builtin/arithmetic.py

@@ -1291,7 +1291,7 @@ def test(self, expr) -> bool:
 class RealSign(Builtin):
     """
     <url>
-        :Signum:
+        :Sign function:
         https://en.wikipedia.org/wiki/Sign_function</url> (<url>
         :WMA link:
         https://reference.wolfram.com/language/ref/RealSign.html