Makefile: fix doc target

meson generated didn't generate the `doc` target because it appended
the output directory to the target name. This commit sets the output
name to `.`, so that the target is named `doc` instead of `doc/html`.

fixes #75.

Makefile

@@ -5,7 +5,7 @@
 ######################################################################
 
 NINJA_TARGETS := test benchmark install dist scan-build clang-format uninstall \
-	all clang-tidy doc coverage-html
+	all clang-tidy doc/html coverage-html
 
 ifeq ($(32BIT),1)
 	ARCH = i386
@@ -65,6 +65,10 @@ endif
 $(NINJA_TARGETS): $(BUILD_DIR)
 	$W $(NINJA) -C $< $@
 
+.PHONY: doc
+
+doc: doc/html
+
 $(BUILD_DIR): meson.build Makefile
 	[ "$(PODMAN)" ] && meson wrap update-db || true
 	[ -d "$@" ] && rm -rf "$@" || true