Fix typo on GitHub sign-in button (#518) (#519)

Author: t0rr3sp3dr0

soy/elements.soy

@@ -552,7 +552,7 @@
     {case 'google.com'}
       {msg desc="Display name of the identity provider for Google sign-in."}Google{/msg}
     {case 'github.com'}
-      {msg desc="Display name of the identity provider for Github sign-in."}Github{/msg}
+      {msg desc="Display name of the identity provider for GitHub sign-in."}GitHub{/msg}
     {case 'facebook.com'}
       {msg desc="Display name of the identity provider for Facebook sign-in."}Facebook{/msg}
     {case 'twitter.com'}