Update: Add MIC specification for primary exchange (#436)

This PR updates this param text to align with our documentation on the website.

Author: justinpolygon

polygon/rest/reference.py

@@ -99,7 +99,7 @@ def list_tickers(
         :param ticker_gte: Ticker greater than or equal to.
         :param type: Specify the type of the tickers. Find the types that we support via our Ticker Types API. Defaults to empty string which queries all types.
         :param market: Filter by market type. By default all markets are included.
-        :param exchange: Specify the primary exchange of the asset in the ISO code format. Find more information about the ISO codes at the ISO org website. Defaults to empty string which queries all exchanges.
+        :param exchange: Specify the assets primary exchange Market Identifier Code (MIC) according to ISO 10383. Defaults to empty string which queries all exchanges.
         :param cusip: Specify the CUSIP code of the asset you want to search for. Find more information about CUSIP codes at their website. Defaults to empty string which queries all CUSIPs.
         :param cik: Specify the CIK of the asset you want to search for. Find more information about CIK codes at their website. Defaults to empty string which queries all CIKs.
         :param date: Specify a point in time to retrieve tickers available on that date. Defaults to the most recent available date.