Enforcing consistency, 'gitian' to 'Gitian'

There was once instance of 'gitian'. I changed it to 'Gitian' so that it would be consistent with all other instances in the file.

Author: S. Matthew English

contrib/README.md

@@ -38,7 +38,7 @@ Scripts and notes for Mac builds.
 RPM spec file for building bitcoin-core on RPM based distributions
 
 ### [Gitian-build](/contrib/gitian-build.sh) ###
-Script for running full gitian builds.
+Script for running full Gitian builds.
 
 Test and Verify Tools 
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