Fix typo

INSTALL.md

@@ -10,7 +10,7 @@ then follow these steps:
 1. Clone your fork of the repo
 1. Install the Template Toolkit using your favourite CPAN module installation technique
 1. Make whatever changes you want
-1. Build the site locally using `ttree -f ttreerrc`
+1. Build the site locally using `ttree -f ttreerc`
 1. Run a local web server (I like [`http_this`](https://metacpan.org/pod/App::HTTPThis) to test your changes
 1. When you're happy, commit your changes and push them to your fork
 1. Create a [pull request](https://github.com/LondonPM/London-pm-website/pulls) to merge your changes to the main repo