Fix a FTBFS with sphinx 7 (#1042642)
Thanks to s3v for the patch.

--- a/ns-3.40/doc/manual/source/conf.py
+++ b/ns-3.40/doc/manual/source/conf.py
@@ -65,7 +65,7 @@
 
 # General information about the project.
 project = u'ns-3 project'
-copyright = u'2006-2019'
+copyright = u'2006-2019, ns-3 project'
 #author = u'test'
 
 # The version info for the project you're documenting, acts as replacement for
--- a/ns-3.40/doc/models/source/conf.py
+++ b/ns-3.40/doc/models/source/conf.py
@@ -65,7 +65,7 @@
 
 # General information about the project.
 project = u'ns-3 project'
-copyright = u'2006-2019'
+copyright = u'2006-2019, ns-3 project'
 #author = u'test'
 
 # The version info for the project you're documenting, acts as replacement for
--- a/ns-3.40/doc/tutorial/source/conf.py
+++ b/ns-3.40/doc/tutorial/source/conf.py
@@ -63,7 +63,7 @@
 
 # General information about the project.
 project = u'ns-3 project'
-copyright = u'2006-2019'
+copyright = u'2006-2019, ns-3 project'
 #author = u'test'
 
 # The version info for the project you're documenting, acts as replacement for
