Newcomers_Guide.rst: Fix promotion link markup
Newcomers_Guide.rst: Fix promotion link markup

This corrects the typo in markup
by adding '_' after the Google promotion link.

Closes https://github.com/coala/coala/issues/4880

karansingh1559/coala,coala/coala,shreyans800755/coala,coala/coala,jayvdb/coala,jayvdb/coala,SanketDG/coala,Nosferatul/coala,coala-analyzer/coala,shreyans800755/coala,SanketDG/coala,shreyans800755/coala,jayvdb/coala,karansingh1559/coala,coala-analyzer/coala,Nosferatul/coala,SanketDG/coala,Nosferatul/coala,coala-analyzer/coala,karansingh1559/coala,coala/coala