[DOCS] Fix docs update instruction (#273)

Author: aobolensk

docs/Makefile

@@ -16,6 +16,9 @@ help:
 
 .PHONY: help update Makefile
 
+gettext:
+	@$(SPHINXBUILD) -b gettext "$(SOURCEDIR)" _build/gettext $(SPHINXOPTS)
+
 update:
 	@$(SPHINXINTL) update -p _build/gettext -l en -l ru
 

docs/README.md

@@ -22,8 +22,10 @@ pip install -r requirements.txt
 make html
 ```
 
-5. Update the locale files:
+5. Update the documentation:
 ```bash
+make gettext
+# update documentation
 make update
 ```