[Docs] Update README for install notes

* Changes the suggested shell when running the install script to $SHELL
  which will return the default shell.
* Adds a note for how to fix the issue when the install script is not
  updating the correct profile file.

README.md

@@ -59,13 +59,13 @@ nvm is a version manager for [node.js](https://nodejs.org/en/), designed to be i
 
 To **install** or **update** nvm, you should run the [install script][2]. To do that, you may either download and run the script manually, or use the following cURL or Wget command:
 ```sh
-curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.35.3/install.sh | bash
+curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.35.3/install.sh | $SHELL
 ```
 ```sh
-wget -qO- https://raw.githubusercontent.com/nvm-sh/nvm/v0.35.3/install.sh | bash
+wget -qO- https://raw.githubusercontent.com/nvm-sh/nvm/v0.35.3/install.sh | $SHELL
 ```
 
-Running either of the above commands downloads a script and runs it. The script clones the nvm repository to `~/.nvm`, and attempts to add the source lines from the snippet below to the correct profile file (`~/.bash_profile`, `~/.zshrc`, `~/.profile`, or `~/.bashrc`).
+Running either of the above commands downloads a script and runs it. The script clones the nvm repository to `~/.nvm`, and attempts to add the source lines from the snippet below to the correct profile file (`~/.bashrc`, `~/.bash_profile`, `~/.zshrc`, or `~/.profile`). If you find the install script is updating the wrong profile file, set the `$PROFILE` env var to the profiles file path, and then rerun the installation script.
 
 <a id="profile_snippet"></a>
 ```sh