Update README.md "Updating Poetry"

Currently the note in "Updating Poetry" is different from the one below in "Enable tab completion for Bash, Fish, or Zsh". This MR is to make them more consistent.

README.md

@@ -65,7 +65,7 @@ to `self update`.
 poetry self update 1.0.0
 poetry self update 1.0.0
 ```
 ```
 
 
-!!!note
+*Note:*
 
 
     If you are still on poetry version < 1.0 use `poetry self:update` instead.
     If you are still on poetry version < 1.0 use `poetry self:update` instead.