Add example for Dir.glob (#15171)

src/dir/glob.cr

@@ -37,6 +37,10 @@ class Dir
 
   # Returns an array of all files that match against any of *patterns*.
   #
+  # ```
+  # Dir.glob "path/to/folder/*.txt" # Returns all files in the target folder that end in ".txt".
+  # Dir.glob "path/to/folder/**/*"  # Returns all files in the target folder and its subfolders.
+  # ```
   # The pattern syntax is similar to shell filename globbing, see `File.match?` for details.
   #
   # NOTE: Path separator in patterns needs to be always `/`. The returned file names use system-specific path separators.