Use $HOME instead of tilde inside quotes

The tilde character is not expanded when inside quotes. See https://github.com/robbyrussell/oh-my-zsh/issues/4668#issuecomment-162213379

README.markdown

@@ -104,7 +104,7 @@ The default location is `~/.oh-my-zsh` (hidden in your home directory)
 If you'd like to change the install directory with the `ZSH` environment variable, either by running `export ZSH=/your/path` before installing, or by setting it before the end of the install pipeline like this:
 
 ```shell
-export ZSH="~/.dotfiles/oh-my-zsh"; sh -c "$(curl -fsSL https://raw.github.com/robbyrussell/oh-my-zsh/master/tools/install.sh)"
+export ZSH="$HOME/.dotfiles/oh-my-zsh"; sh -c "$(curl -fsSL https://raw.github.com/robbyrussell/oh-my-zsh/master/tools/install.sh)"
 ```
 
 #### Manual Installation