Add an entry to doc/news/changes/minor to summarixes the change for step-51 about...

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+Changed: 
+1. Add the HDG overview article in the references.bib file.The overview article is by N.C. Nguygen and J. Peraire: Hybirdzable discontinuous Galerkin methods for partial differential equations in continuum mechanics, Journal of Computational Physics, 2012.  
+2. In the introduction part of step-51, I replace the above article by its referenced name.
+<br>
+(Jiuhua Hu, 2019/08/07)
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