Correct require path in readme.md for version 1.0.6

[poudelprakash]

The example as currently written does not work in the latest version(1.0.6) of gem; This fixes

Here is the sample code I ran on my machine:

$ irb --simple-prompt
>> require 'whatlanguage'
=> true
>> "This is a test".language 
NoMethodError: undefined method `language' for "This is a test":String
    from (irb):2
    from /home/prakash/.rvm/rubies/ruby-2.3.0/bin/irb:11:in `<main>'
>> require 'whatlanguage/string'
=> true
>> "This is a test".language 
=> :polish