Fix title of README according to Markdown conventions

The first heading in some README's are indicated using a second level
heading (`##`), which in my opinion is of incorrect structure.
Therefore, in this patch I changed the first heading to a first level
heading (`#`) in README's where this incorrect structure occurs.

[ci skip]

README.md

@@ -1,4 +1,4 @@
-## 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

@@ -1,4 +1,4 @@
-## README
+# README
 
 This README would normally document whatever steps are necessary to get the
 application up and running.

tools/README.md

@@ -1,4 +1,4 @@
-## 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.