Change "River of tiddlers" to "Static HTML tiddlers"

For clarity; see this discussion:

https://groups.google.com/d/msg/tiddlywiki/hReAZYepG5U/bQt8McdkQMUJ

core/language/en-GB/Exporters.multids

@@ -1,6 +1,6 @@
 title: $:/language/Exporters/
 
-StaticRiver: River of tiddlers as static HTML file
+StaticRiver: Static HTML tiddlers
 JsonFile: JSON tiddlers file
 CsvFile: CSV tiddlers file
 TidFile: Single tiddler ".tid" file