build+ci: sync with ndn-cxx

Change-Id: Iff3ad63469eccb47571ba4abd3d42936f0cd8650
diff --git a/docs/INSTALL.rst b/docs/INSTALL.rst
index b2a5a09..22e20d4 100644
--- a/docs/INSTALL.rst
+++ b/docs/INSTALL.rst
@@ -12,7 +12,7 @@
 
 - doxygen
 - graphviz
-- sphinx >= 4.0
+- sphinx
 - sphinxcontrib-doxylink
 
 Build