Merge pull request #834 from snnz/fix-gridtables

Fix an incorrect offset in GridTableParser.

src/Markdig/Extensions/Tables/GridTableParser.cs

@@ -256,7 +256,7 @@ private BlockState HandleContents(BlockProcessor processor, GridTableState table
                 {
                     sliceForCell.End = line.Start + columnEnd - 1;
                 }
-                else if (line.PeekCharExtra(line.End) == '|')
+                else if (line.PeekCharExtra(line.End - line.Start) == '|')
                 {
                     sliceForCell.End = line.End - 1;
                 }