man: improve compatibility with OpenBSD

doc/Makefile.am

@@ -190,7 +190,7 @@ $(MANPAGES_IN): $(MANPAGES_RST)
 	$(AM_V_SPHINX)$(SPHINXBUILD) -b man -d $(SPHINXBUILDDIR)/doctrees $(man_SPHINXOPTS) $(SPHINXBUILDDIR)/man
 	mkdir -p $(srcdir)/man
 	for f in $(SPHINXBUILDDIR)/man/*; do \
-	  sed -e '/^.TP$$/{n;s/^.B //}' "$$f" > "$(srcdir)/man/$$(basename $$f)in"; \
+	  sed -e '/^\.TP$$/ {' -e 'n' -e 's/^\.B //' -e '}' "$$f" > "$(srcdir)/man/$$(basename $$f)in"; \
 	done
 	$(AM_V_ECHO)
 	$(AM_V_ECHO) "Build finished. The man pages are in man/."