fix visual bug resulting from 'TextBuilder.buildTextStringSearch' function. When we have a search string 'abc' and a file 'abcabc', there are two matches and it should also look like there are two matches, but it did not look like there were two matches prior to this commit. This commit fixes that by advancing the search position before adding to the text accumulator.

shell/shell.sml

@@ -69,7 +69,7 @@ struct
         let
           val buffer = #buffer app
           val buffer = LineGap.goToStart buffer
-          val searchString = "val "
+          val searchString = "abc"
           val searchList = SearchList.build (buffer, searchString)
           val buffer = LineGap.goToStart buffer
         in