[Doc] Fix typos

Thanks @segiddins.

lib/cocoapods/generator/module_map.rb

 module Pod
   module Generator
     # Generates LLVM module map files. A module map file is generated for each
-    # Pod and for each Pod target definition, that is built as framework. It
-    # specifies a different umbrella header then usual to avoid name conflicts
+    # Pod and for each Pod target definition that is built as framework. It
+    # specifies a different umbrella header than usual to avoid name conflicts
     # with existing headers of the podspec.
     #
     class ModuleMap