Update README.md

Fix missing `:` and misspelt argument. Makes this example work.

README.md

@@ -52,7 +52,7 @@ require 'bootsnap'
 env = ENV['RAILS_ENV'] || "development"
 Bootsnap.setup(
  cache_dir: 'tmp/cache', # Path to your cache
- ignored_directories ['node_modules'], # Directory names to skip.
+ ignore_directories: ['node_modules'], # Directory names to skip.
  development_mode: env == 'development', # Current working environment, e.g. RACK_ENV, RAILS_ENV, etc
  load_path_cache: true, # Optimize the LOAD_PATH with a cache
  compile_cache_iseq: true, # Compile Ruby code into ISeq cache, breaks coverage reporting.