Update the "Literal Patterns" section

Author: johnslavik

Doc/reference/compound_stmts.rst

@@ -858,7 +858,7 @@ A literal pattern corresponds to most
                   : | "None"
                   : | "True"
                   : | "False"
-   signed_number: ["-"] NUMBER
+   signed_number: ["+" | "-"] NUMBER
 
 The rule ``strings`` and the token ``NUMBER`` are defined in the
 :doc:`standard Python grammar <./grammar>`.  Triple-quoted strings are