doc: fix broken text search

Require Sphinx<3.4.0 to avoid the issue described in:

sphinx-doc/sphinx#8603

This requirement can be relaxed once a new sphinx-rtd-theme is
released.

Signed-off-by: Fabio Utzig <[email protected]>

Author: utzig

scripts/requirements-doc.txt

@@ -2,7 +2,7 @@
 
 breathe>=4.23.0
 docutils>=0.16
-sphinx>=3.3.0,<4.0
+sphinx>=3.3.0,<3.4.0
 sphinx_rtd_theme
 sphinx-tabs
 sphinxcontrib-svg2pdfconverter