Update pow.xhtml

Added missing `0` in Description.

sl4/pow.xhtml

@@ -58,7 +58,7 @@
         <p>
             <code class="function">pow</code> returns the value of <em class="parameter"><code>x</code></em> raised to the <em class="parameter"><code>y</code></em> power. i.e.,
             <math overflow="scroll"><msup><mi>x</mi><mi>y</mi></msup></math>.
-            Results are undefined if <math overflow="scroll"><mi>x</mi><mo>&lt;</mo></math>
+            Results are undefined if <math overflow="scroll"><mi>x</mi><mo>&lt; 0</mo></math>
             or if <math overflow="scroll"><mi>x</mi><mo>=</mo><mn>0</mn></math> and
             <math overflow="scroll"><mi>y</mi><mo>=</mo><mn>0</mn></math>.
         </p>