Update README [ci skip]

Author: hidakatsuya

README.md

@@ -95,7 +95,7 @@ Options:
   -q, [--quiet], [--no-quiet], [--skip-quiet]        # Minimal output
 ```
 
-### rex install [ENV]
+### rex install [env]
 
 Installs extensions in the specified ENV environment and makes them available for use. Specifically, it does the following:
 
@@ -106,7 +106,7 @@ If the specified ENV is currently installed, it compares the current `.extension
 * Uninstalls deleted extensions (the `uninstalled` hook is executed).
 * Reload extensions whose source settings has changed (for example, the `branch` or `tag` has changed) and runs the database migration if necessary.
 
-### rex update
+### rex update [extensions...]
 
 Loads `.extensions.lock` and updates the currently installed extensions to the latest version. `.extensions.rb` is NOT referenced in this command.