change sphix build directory to _build

Author: willforde

docs/Makefile

@@ -6,7 +6,7 @@ SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
 SPHINXPROJ    = HTMLement
 SOURCEDIR     = .
-BUILDDIR      = build
+BUILDDIR      = _build
 
 # Put it first so that "make" without argument is like "make help".
 help:
@@ -17,4 +17,4 @@ help:
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)