Change the back-cover text.

c.texi

@@ -29,8 +29,7 @@ Texts as in (a) below.  A copy of the license is included in the
 section entitled ``GNU Free Documentation License.''
 
 (a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
-modify this GNU manual.  Buying copies from the FSF supports it in
-developing GNU and promoting software freedom.''
+modify this GNU manual.''
 @end quotation
 @end copying