Docs: Fix sphinxcontrib dependencies

docs/requirements.txt

@@ -8,6 +8,9 @@ sphinxcontrib-blockdiag>=1.5.5, <2.0.0
 sphinxcontrib-seqdiag>=0.8.5, <2.0.0
 sphinxcontrib-actdiag>=0.8.5, <2.0.0
 sphinxcontrib-nwdiag>=0.9.5, <2.0.0
-nwdiag==1.0.4
+blockdiag>=1.5.4, <2.0.0
+seqdiag>=0.9.6, <2.0.0
+actdiag>=0.5.4, <2.0.0
+nwdiag>=1.0.4, <2.0.0
 recommonmark
 future>=0.16.0 # for ../tools/gen_esp_err_to_name.py