Update README.md for plantuml exporter

Removed the referenced repository that is archived and deprecated.

Author: beliaev-maksim

structurizr-export/src/main/java/com/structurizr/export/plantuml/README.md

@@ -2,6 +2,4 @@
 
 There are two PlantUML exporters in this package - [StructurizrPlantUMLExporter](StructurizrPlantUMLExporter.java) and [C4PlantUMLExporter](C4PlantUMLExporter.java).
 
-If neither of these provide the features you are looking for, an alternative PlantUML exporter can be found at [https://github.com/cloudflightio/structurizr-export-c4plantuml](https://github.com/cloudflightio/structurizr-export-c4plantuml).
-
-See https://docs.structurizr.com/export for more.
+See https://docs.structurizr.com/export for more.