fix typo error "description"

Author: pakuland3

tools/tensorflow_docs/api_generator/doc_generator_visitor.py

@@ -170,7 +170,7 @@ def __init__(self):
     This object accumulates the various data-structures necessary to build the
     docs, including (see the property definitions for details.):
 
-    In the decsription below "main name" is the object's preferred fully
+    In the description below "main name" is the object's preferred fully
     qualified name.
 
     Params: