Added a link to "spaceship operator"

Author: SakiTakamachi

reference/bc/bcmath/number/compare.xml

@@ -14,7 +14,7 @@
   </methodsynopsis>
   <simpara>
    Compare two arbitrary precision numbers.
-   This method behaves similar to the spaceship operator.
+   This method behaves similar to the <link linkend="language.operators.comparison">spaceship operator</link>.
   </simpara>
  </refsect1>