Fix source_files file path in podspec

The addition of the Swift Package file changed the file path for the math parser's source files. This change updates the podspec with the new source_files file path.

Author: graycampbell

DDMathParser.podspec

@@ -16,7 +16,7 @@ Pod::Spec.new do |s|
   s.osx.deployment_target = '10.9'
   s.watchos.deployment_target = '2.0'
 
-  s.source_files = 'MathParser/*.{h,m,swift}'
+  s.source_files = 'MathParser/Sources/MathParser/*.{h,m,swift}'
 
   s.requires_arc = true
   s.module_name = 'MathParser'