small concepts fixes

Author: AndreiKingsley

docs/StardustDocs/d.tree

@@ -21,7 +21,7 @@
         <toc-element topic="gettingStartedDatalore.md"/>
         <toc-element topic="gettingStartedGradleAdvanced.md"/>
     </toc-element>
-    <toc-element topic="concepts.md">
+    <toc-element topic="concepts.md" accepts-web-file-names="overview.html">
         <toc-element topic="apiLevels.md"/>
         <toc-element topic="types.md">
             <toc-element topic="DataFrame.md"/>

docs/StardustDocs/topics/concepts/concepts.md

@@ -18,9 +18,9 @@ understand key concepts, motivation, and the overall structure of the library.
 <show-structure depth="3"/>
 
 
-## What is Dataframe
+## What is a dataframe
 
-Dataframe is an abstraction for working with structured data. 
+A *dataframe* is an abstraction for working with structured data. 
 Essentially, it’s a 2-dimensional table with labeled columns of potentially different types. 
 You can think of it like a spreadsheet or SQL table, or a dictionary of series objects.