Fix TagMap link (#239)

Author: mayurkale22

encodings/BinaryEncoding.md

@@ -139,7 +139,7 @@ https://developers.google.com/protocol-buffers/docs/encoding#varints.
 * Multiple tag fields can contain the same tag key. All but the last value for
   that key should be ignored.
 * The
-  [size limit for serialized Tag Contexts](https://github.com/census-instrumentation/opencensus-specs/blob/master/tags/TagContext.md#serialization)
+  [size limit for serialized Tag Contexts](https://github.com/census-instrumentation/opencensus-specs/blob/master/tags/TagMap.md#limits)
   should apply to all tag fields, even if some of them have duplicate keys. For
   example, a serialized tag context with 10,000 small tags that all have the
   same key should be considered too large.

tags/README.md

@@ -3,4 +3,4 @@ This documentation serves to document the "look and feel" of the open source tag
 describes the key types and the overall behavior.
 
 ## Main APIs
-* [TagContext](TagContext.md)
+* [TagMap](TagMap.md)