Update task status and finalize MarkdownFormatter implementation

Author: jbrinkman

.kiro/specs/dotnet-api-diff/tasks.md

@@ -123,7 +123,7 @@ Each task should follow this git workflow:
     - **Git Workflow**: Create branch `feature/task-6.2-json-formatter`, commit, push, and create PR
     - _Requirements: 4.1, 4.2_
 
-  - [ ] 6.3 Implement markdown output formatter
+  - [x] 6.3 Implement markdown output formatter
     - Create MarkdownFormatter generating human-readable documentation
     - Include tables, sections for different change types, and exclusions
     - Write unit tests for markdown formatting and structure

src/DotNetApiDiff/Reporting/MarkdownFormatter.cs

@@ -172,8 +172,6 @@ private void FormatChangeGroup(StringBuilder output, List<ApiDifference> changes
 
                     output.AppendLine("</details>");
                 }
-
-                output.AppendLine();
             }
 
             output.AppendLine();