Adds missing minor doc about tools/test.rb [ci skip]

... and minor typo fix

tools/README.md

@@ -6,3 +6,5 @@ They aren't used by Rails apps directly.
   * `console` drops you in irb and loads local Rails repos
   * `profile` profiles `Kernel#require` to help reduce startup time
   * `line_statistics` provides CodeTools module and LineStatistics class to count lines
+  * `test` is loaded by every major component of Rails to simplify testing, for example:
+    `cd ./actioncable; bin/test ./path/to/actioncable_test_with_line_number.rb:5`

tools/test.rb

@@ -8,7 +8,7 @@ require "rails/test_unit/line_filtering"
 require "active_support/test_case"
 
 class << Rails
-  # Necessary to get rerun-snippts working.
+  # Necessary to get rerun-snippets working.
   def root
     @root ||= Pathname.new(COMPONENT_ROOT)
   end