fix a very small typo

Author: tenderlove

README.md

@@ -161,7 +161,7 @@ and run `bundle install` from your shell.
 Potential performance improvements may be achieved under MRI by installing optional C extensions.
 To minimize installation errors the C extensions are available in the `concurrent-ruby-ext` extension
 gem. `concurrent-ruby` and `concurrent-ruby-ext` are always released together with same version.
-Simply install the extension gen too:
+Simply install the extension gem too:
 
 ```ruby
 gem install concurrent-ruby-ext