doc/Makefile: Remove superfluous quotation marks fixing syntax error

Change-Id: I4c269248a3a1e078204882d04964fd5af4b5f51a
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75799
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Daniel Maslowski <info@orangecms.org>
diff --git a/Documentation/Makefile b/Documentation/Makefile
index 19b1f80..60a3f35 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -45,7 +45,7 @@
 	$(PDFLATEX) corebootBuildingGuide.tex
 
 sphinx: $(SPHINXDIR)
-	$(MAKE) -f Makefile.sphinx html BUILDDIR="$(SPHINXDIR)""
+	$(MAKE) -f Makefile.sphinx html BUILDDIR="$(SPHINXDIR)"
 
 clean-sphinx:
 	$(MAKE) -f Makefile.sphinx clean BUILDDIR="$(SPHINXDIR)"