Add CSS rule for c-Singleline

Due a bug in the Perl 6 pygments lexer, single line comments
are given a CSS class that doesn't exist in the normal set
of classes that pygments provides.  This will allow single-line
comments to be highlighted until that bug is fixed

html/css/pygments.css

@@ -4,6 +4,7 @@ pre { line-height: 125%; }
 body .hll { background-color: #ffffcc }
 body  { background: #f8f8f8; }
 body .c { color: #408080; font-style: italic } /* Comment */
+body .c-Singleline { color: #408080; font-style: italic } /* Comment */
 body .err { border: 1px solid #FF0000 } /* Error */
 body .k { color: #008000; font-weight: bold } /* Keyword */
 body .o { color: #666666 } /* Operator */