ccmake: Append rather than replace Generators section of docs

Using SetSection() discards the heading line populated by
addCMakeStandardDocSections(), whereas AppendSection()
preserves it. This also makes the code used for ccmake
consistent with that used for cmake and cmake-gui.

Source/CursesDialog/ccmake.cxx

@@ -95,7 +95,7 @@ int main(int argc, char const* const* argv)
     if (argc == 1) {
       doc.AppendSection("Usage", cmDocumentationUsageNote);
     }
-    doc.SetSection("Generators", generators);
+    doc.AppendSection("Generators", generators);
     doc.PrependSection("Options", cmDocumentationOptions);
     return doc.PrintRequestedDocumentation(std::cout) ? 0 : 1;
   }