Fix sphinx-toolbox version in requirements.txt (#920)

Signed-off-by: Denisa <denisa@eprosima.com>

Author: depink5

docs/requirements.txt

@@ -17,6 +17,6 @@ sphinxcontrib-plantuml==0.29
 sphinxcontrib-qthelp==1.0.7
 sphinxcontrib-serializinghtml==1.1.10
 sphinxcontrib.spelling==8.0.0
-sphinx-toolbox==3.7.0
+sphinx-toolbox==3.8.0
 vcstool==0.3.0
 xmlschema==3.3.2