remove confusing comma

Bob DuCharme's 7th comment
https://www.w3.org/mid/[email protected]

Bob notes that the comma makes it look like we are introducing a new idea

Author: pchampin

spec/index.html

@@ -176,7 +176,7 @@ <h3>Resources and Statements</h3>
     <a>datatypes</a> that define the range of possible
     values, such as strings, numbers, and dates. Special kinds of literals &mdash;
     <a>language-tagged strings</a> and <a>directional language-tagged strings</a> &mdash;
-    respectively denote plain-text strings in a natural language, and plain-text
+    respectively denote plain-text strings in a natural language and plain-text
     strings in a natural language including an initial text direction.</p>
 
     <p>Asserting an <a>RDF triple</a> says that <em>some relationship,