docs: Clarify oh-my-zsh installation instructions.

Thanks to Andrew Janke.

Fixes zsh-users/zsh-syntax-highlighting#270.

INSTALL.md

@@ -42,7 +42,7 @@ To install zsh-syntax-highlighting under oh-my-zsh:
 
 1. Clone this repository in oh-my-zsh's plugins directory:
 
-        git clone https://github.com/zsh-users/zsh-syntax-highlighting.git ~/.oh-my-zsh/custom/plugins/zsh-syntax-highlighting
+        git clone https://github.com/zsh-users/zsh-syntax-highlighting.git ${ZSH_CUSTOM:-~/.oh-my-zsh/custom}/plugins/zsh-syntax-highlighting
 
 2. Activate the plugin in `~/.zshrc`: