Use __dir__ in collapse examples [Closes #131]

It is safer to use absolute paths in examples. That way, users do not
accidentally pass relative paths in situations in which the cwd at runtime is
out of the control of the project, as it typically happens with gems.

README.md

@@ -202,7 +202,7 @@ booking/actions/create.rb -> Booking::Create
 To make it work that way, configure Zeitwerk to collapse said directory:
 
 ```ruby
-loader.collapse("booking/actions")
+loader.collapse("#{__dir__}/booking/actions")
 ```
 
 This method accepts an arbitrary number of strings or `Pathname` objects, and also an array of them.
@@ -212,7 +212,7 @@ You can pass directories and glob patterns. Glob patterns are expanded when they
 To illustrate usage of glob patterns, if `actions` in the example above is part of a standardized structure, you could use a wildcard:
 
 ```ruby
-loader.collapse("*/actions")
+loader.collapse("#{__dir__}/*/actions")
 ```
 
 <a id="markdown-nested-root-directories" name="nested-root-directories"></a>