fix missing space around operator style

This is an error in PEP8 but not in pylint.

python/pyspark/rdd.py

@@ -708,7 +708,7 @@ def pipe_objs(out):
             pipe.wait()
             if pipe.returncode:
                 raise Exception("Pipe function `%s' exited "
-                                "with error code %d" %(command, pipe.returncode))
+                                "with error code %d" % (command, pipe.returncode))
             return result
         return self.mapPartitions(func)