Merge pull request #80 from YoshinoriN/issue/79

chore(doc): update modifyAnchors explanation (#79)

README.md

@@ -39,7 +39,7 @@ marked:
 - **breaks** - Enable GFM [line breaks](https://help.github.com/articles/github-flavored-markdown#newlines). This option requires the `gfm` option to be true.
 - **smartLists** - Use smarter list behavior than the original markdown.
 - **smartypants** - Use "smart" typograhic punctuation for things like quotes and dashes.
-- **modifyAnchors** - Use for transform anchorIds. if 1 to lowerCase and if 2 to upperCase.
+- **modifyAnchors** - Use for transform anchorIds. if `1` to lowerCase and if `2` to upperCase. **Must be integer**.
 - **autolink** - Enable autolink for URLs. E.g. `https://hexo.io` will become `<a href="https://hexo.io">https://hexo.io</a>`.
 
 ## Extras