chore:fix cargo clippy warning

[xixishidibei]

Running cargo clippy reports the following error.

    Checking jh v0.2.0-pre (/Users/xxx/yyyyy/hashes/jh)
warning: doc list item overindented
  --> sha3/src/../README.md:21:1
   |
21 |    without padding changes)
   | ^^^ help: try using `  ` (2 spaces)
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items
   = note: `#[warn(clippy::doc_overindented_list_items)]` on by default

This commit resolves the issue.

[newpavlov]

It will be fixed in #678.