remove usesless prompt dollar sign from install commands

Author: gturi

README.md

@@ -26,17 +26,17 @@ CompletablePromise allows to create a Promise instance that does not start its r
 
 Using npm:
 ```
-$ npm install completable-promise
+npm install completable-promise
 ```
 
 Using bower:
 ```
-$ bower install completable-promise
+bower install completable-promise
 ```
 
 Using yarn:
 ```
-$ yarn add completable-promise
+yarn add completable-promise
 ```
 
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