Fix for static analysis

[bbirman]

I got this error on a run and I think it's just a small typo (haven't tried testing on a fork):

[!] Invalid `StaticAnalysis.rb` file: undefined local variable or method `issue' for an instance of Danger::Dangerfile
 #  from StaticAnalysis.rb:29
 #  -------------------------------------------
 #          issues = report['diagnostics']
 >          print "File modified in PR: #{file}, has #{issue.count} issues.\n"
 #          for i in 0..issues.count-1
 #  -------------------------------------------