added '/' at the end of the ~/bin path

I added a forward slash at the end of the ~/bin path so it will become this: ~/bin/
This is to prevent people from renaming tmpmail to ~/bin and causing confusion later on

README.md

@@ -28,7 +28,7 @@ use the `--browser` argument followed by the command needed to launch the web br
 $ curl -L "https://git.io/tmpmail" > tmpmail && chmod +x tmpmail
 
 # Then move it somewhere in your $PATH. Here is an example:
-$ mv tmpmail ~/bin
+$ mv tmpmail ~/bin/
 ```
 
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