Update PluginMechanism.tid (#2574)

Added link to sematic versioning

editions/tw5.com/tiddlers/mechanisms/PluginMechanism.tid

@@ -28,7 +28,7 @@ Plugins are stored as tiddlers with the following fields:
 |title |Title of plugin |
 |description |Description of plugin |
 |author |Author of plugin |
-|version |Version string (must conform to SemanticVersioning convention) |
+|version |Version string (must conform to [ext[SemanticVersioning|http://semver.org/]] convention) |
 |source |Source URL of plugin |
 |type |Must be ''application/json'' |
 |plugin-type |Can be ''plugin'' (default), ''language'' or ''theme'' |