Merge pull request #72 from kay-kim/master

Docs: update installation for bson gem 4.0

Author: estolfo

docs/tutorials/bson-v4.txt

@@ -32,7 +32,7 @@ To install the gem with bundler, include the following in your Gemfile:
 
 .. code-block:: ruby
 
-    gem 'bson', '~> 3.0'
+    gem 'bson', '~> 4.0'
 
 The BSON gem is compatible with MRI 1.9.3, 2.0.x, 2.1.x, 2.2.x, JRuby 1.7.x, and
 Rubinius 2.5.x
@@ -204,7 +204,7 @@ Represents a placeholder for a value that was not provided.
   BSON::Undefined.new
 
 ``BSON::Decimal128``
-```````````````````
+````````````````````
 
 Represents a 128-bit decimal-based floating-point value capable of
 emulating decimal rounding with exact precision..