nit: s/plan/plain/

Change-Id: I5797c46f9fc614d12166b7e1c27b94ed6e86010a
Reviewed-on: https://gerrit.libreoffice.org/31744Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

sw/README

@@ -15,7 +15,7 @@ comments show that Writer core dates back until at least November
 == Source contents ==
  * core: Writer core (document model, layout, UNO API implementation)
  * filter: Writer internal filters
-   * ascii: plan text filter
+   * ascii: plain text filter
    * basflt
    * html: HTML filter
    * inc: include files for filters