Em dashes instead of consecutive hyphens. (matplotlib#25211)

* Em dashes instead of consecutive hyphens.

* Code review: Unicode instead of HTML escapes.

Co-authored-by: Oscar Gustafsson <[email protected]>

* Review comment: spaced dashes.

---------

Co-authored-by: Oscar Gustafsson <[email protected]>

Author: tfpf

README.md

@@ -34,12 +34,11 @@ which is generated from `/doc/users/installing/index.rst`
 
 ## Contribute
 
-You've discovered a bug or something else you want to change -
-excellent!
+You've discovered a bug or something else you want to change — excellent!
 
-You've worked out a way to fix it -- even better!
+You've worked out a way to fix it — even better!
 
-You want to tell us about it -- best of all!
+You want to tell us about it — best of all!
 
 Start at the [contributing
 guide](https://matplotlib.org/devdocs/devel/contributing.html)!