readme: add shell signs

Author: efiop

README.rst

@@ -88,13 +88,13 @@ Homebrew
 
 .. code-block:: bash
 
-   brew install iterative/homebrew-dvc/dvc
+   $ brew install iterative/homebrew-dvc/dvc
 
 or:
 
 .. code-block:: bash
 
-   brew cask install iterative/homebrew-dvc/dvc
+   $ brew cask install iterative/homebrew-dvc/dvc
 
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