Merge pull request #23863 from delftswa2016/fix-readme-heading

Fix README heading according to Markdown conventions

README.md

-## Welcome to Rails
+# Welcome to Rails
 
 Rails is a web-application framework that includes everything needed to
 create database-backed web applications according to the

railties/lib/rails/generators/rails/app/templates/README.md

-## README
+# README
 
 This README would normally document whatever steps are necessary to get the
 application up and running.

tools/README.md

-## Rails dev tools
+# Rails dev tools
 
 This is a collection of utilities used for Rails internal development.
 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
+  * `line_statistics` provides CodeTools module and LineStatistics class to count lines
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