Fix spelling of "cannot" (#885)

Author: deivid-rodriguez

lib/thor/actions/file_manipulation.rb

@@ -348,7 +348,7 @@ def capture(*args)
     end
 
     def with_output_buffer(buf = "".dup) #:nodoc:
-      raise ArgumentError, "Buffer can not be a frozen object" if buf.frozen?
+      raise ArgumentError, "Buffer cannot be a frozen object" if buf.frozen?
       old_buffer = output_buffer
       self.output_buffer = buf
       yield