Updated NEXMO_ references to VONAGE_ in README.md

[GregHolmes]

Due to the Rebrand, references to NEXMO have been updated to Vonage. This includes the placeholders where users are instructed to replace the values with their own unique values

Description

Updated all references to NEXMO_ placeholders with VONAGE_ placeholders

Motivation and Context

This problem continues the process of the SDK rebrand from Nexmo to Vonage

How Has This Been Tested?

No code needs testing, I have viewed the changes through a markdown previewer however.

Example Output or Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #254 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #254   +/-   ##
=========================================
  Coverage     80.31%   80.31%           
  Complexity     2396     2396           
=========================================
  Files           166      166           
  Lines          6072     6072           
=========================================
  Hits           4877     4877           
  Misses         1195     1195           

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