DOC: typo

README.md

@@ -24,7 +24,7 @@ requested by the user.
 
 
 When called like this, the script will download a "manifest" file from a fixed
-URL, which is a JSON file containing a list of all modules that are availble,
+URL, which is a JSON file containing a list of all modules that are available,
 and where they can be downloaded from. Once the manifest file has been
 downloaded, `fsl_add_module` will display a list of all available modules, and
 will prompt the user to select which modules they would like to download.