fix documentation of :noweb strip-export header arg

  Thanks to Christophe Pouzat for catching and reporting this.

* doc/org.texi (noweb): Removed erroneous negative.

doc/org.texi

@@ -14848,7 +14848,7 @@ references will not be expanded when the code block is exported.
 @item @code{strip-export}
 ``Noweb'' syntax references in the body of the code block will be expanded
 before the block is evaluated or tangled.  However, ``noweb'' syntax
-references will not be removed when the code block is exported.
+references will be removed when the code block is exported.
 @item @code{eval}
 ``Noweb'' syntax references in the body of the code block will only be
 expanded before the block is evaluated.