BUILD: fix quoting issue

\"$@\" got malformed and interpreted by the Makefile

src/htm/Makefile.am

@@ -7,8 +7,8 @@ thememan.exe: htm-mono.csproj
 	$(MDTOOL) $(MDTOOL_OPTS) build $<
 
 thememan:
-	@echo "#!/bin/sh" > $@
-	@echo "exec mono /usr/bin/thememan.exe \"$@\"" >> $@
+	@echo '#!/bin/sh' > $@
+	@echo 'exec mono /usr/bin/thememan.exe "$$@"' >> $@
 
 clean-local:
 	rm -f thememan.exe thememan.exe.config thememan.exe.mdb thememan Main.resources