Fix: typo in README.md

The generateManifest.bash script accepts as an argument CHROMIUM and
GECKO, not chromium and gecko.

Change-Id: Ibd4e649dca80bd593ffac6670fd275de97ce69c9
diff --git a/README.md b/README.md
index 211b92f..f651f8b 100644
--- a/README.md
+++ b/README.md
@@ -43,7 +43,7 @@
 Run `make clean` to clean all the release files (this removes the `out` folder, which is where the release files are saved).
 
 ## Testing notes
-When testing the extension during development, you don't have to build the extension each time you want to import an updated version to Chrome/Firefox. Instead, run `bash generateManifest.bash {browser}` once, where `{browser}` is either `chromium` or `gecko`, and this will generate a `manifest.json` file for the specified browser in the `src` directory. Now, you can load the `src` folder directly in the browser in order to import the extension, which removes the need to build it. When the `manifest.gjson` file is modified, you'll have to generate the manifest again.
+When testing the extension during development, you don't have to build the extension each time you want to import an updated version to Chrome/Firefox. Instead, run `bash generateManifest.bash {browser}` once, where `{browser}` is either `CHROMIUM` or `GECKO`, and this will generate a `manifest.json` file for the specified browser in the `src` directory. Now, you can load the `src` folder directly in the browser in order to import the extension, which removes the need to build it. When the `manifest.gjson` file is modified, you'll have to generate the manifest again.
 
 To test translations, you might want to set your browser's locale. This section tells you how to set the locale in [Windows](https://developer.chrome.com/extensions/i18n#testing-win), [Mac OS X](https://developer.chrome.com/extensions/i18n#testing-mac), [Linux](https://developer.chrome.com/extensions/i18n#testing-linux), and [Chrome OS](https://developer.chrome.com/extensions/i18n#testing-chromeos).