Typo source directory in Podspec (#738)

Source directory has been renamed in 18608ead653bfe1e05e3d1f6fb6d34f62cec49bd

SnapKit.podspec

@@ -12,7 +12,7 @@ Pod::Spec.new do |s|
   s.osx.deployment_target = '10.11'
   s.tvos.deployment_target = '10.0'
 
-  s.source_files = 'Source/*.swift'
+  s.source_files = 'Sources/*.swift'
 
   s.swift_version = '5.0'
 end