make: fix make doc error

This seems to fix issue ElementsProject#5941
Certain make version(?) will parse the ')' as function end when not
inside "" but ''.

Changelog-None

doc/Makefile

@@ -212,7 +212,7 @@ doc/index.rst: $(MANPAGES:=.md)
 	@$(call VERBOSE, "genidx $@", \
 	  find doc -maxdepth 1 -name '*\.[0-9]\.md' | \
 	  cut -b 5- | LC_ALL=C sort | \
-	  sed 's/\(.*\)\.\(.*\).*\.md/\1 <\1.\2.md>/' | \
+	  sed "s/\(.*\)\.\(.*\).*\.md/\1 <\1.\2.md>/" | \
 	  python3 devtools/blockreplace.py doc/index.rst manpages --language=rst --indent "   " \
 	)