Fix line number in the "Link to Crossdart" link

It used to point to the beginning of the comment section, and also was
off by 1 (I guess because `LineNumberCache` counts them from 0, but
www.crossdart.info starts from 1).

Now it points to the actual declaration.

Verified on http package, e.g.:
https://www.dartdocs.org/documentation/http/0.11.3%2B7/http/BaseClient/head.html

Author: astashov

lib/src/model.dart

@@ -2025,8 +2025,10 @@ abstract class SourceCodeMixin {
   int get _lineNumber {
     var node = element.computeNode();
     if (node is Declaration && (node as Declaration).element != null) {
-      return lineNumberCache.lineNumber(
-          (node as Declaration).element.source.fullName, node.offset);
+      var element = (node as Declaration).element;
+      var lineNumber = lineNumberCache.lineNumber(
+          element.source.fullName, element.nameOffset);
+      return lineNumber + 1;
     } else {
       return null;
     }