user_guide: Fix typo in MultiWayIf chapter

Close #24829

docs/users_guide/exts/multiway_if.rst

@@ -57,5 +57,5 @@ Note that multi-way if supports guards other than boolean conditions: ::
       if | parseNumbers settings
          , Just (exponent, mantissa) <- decomposeNumber str
          , let (integralPart, fractionPart) = parse mantissa
-         , integralPart >= 0 = ...
-         | otherwise = ...
+         , integralPart >= 0 -> ...
+         | otherwise -> ...