[ci skip] Prefix class name with Rails:: in docs

After renaming and deprecation of SourceAnnotationExtractor
documentation has been updated to reflect the new name Rails::SourceAnnotationExtractor

railties/lib/rails/source_annotation_extractor.rb

@@ -29,7 +29,7 @@ def self.directories
       end
 
       # Registers additional directories to be included
-      #   SourceAnnotationExtractor::Annotation.register_directories("spec", "another")
+      #   Rails::SourceAnnotationExtractor::Annotation.register_directories("spec", "another")
       def self.register_directories(*dirs)
         directories.push(*dirs)
       end
@@ -39,7 +39,7 @@ def self.extensions
       end
 
       # Registers new Annotations File Extensions
-      #   SourceAnnotationExtractor::Annotation.register_extensions("css", "scss", "sass", "less", "js") { |tag| /\/\/\s*(#{tag}):?\s*(.*)$/ }
+      #   Rails::SourceAnnotationExtractor::Annotation.register_extensions("css", "scss", "sass", "less", "js") { |tag| /\/\/\s*(#{tag}):?\s*(.*)$/ }
       def self.register_extensions(*exts, &block)
         extensions[/\.(#{exts.join("|")})$/] = block
       end
@@ -69,7 +69,7 @@ def to_s(options = {})
     #
     # Directories may also be explicitly set using the <tt>:dirs</tt> key in +options+.
     #
-    #   SourceAnnotationExtractor.enumerate 'TODO|FIXME', dirs: %w(app lib), tag: true
+    #   Rails::SourceAnnotationExtractor.enumerate 'TODO|FIXME', dirs: %w(app lib), tag: true
     #
     # If +options+ has a <tt>:tag</tt> flag, it will be passed to each annotation's +to_s+.
     #