ec2_customer_gateway: fix bgp_asn documentation inconsistency (#1197)

ec2_customer_gateway: fix bgp_asn documentation inconsistency

SUMMARY

Fixes #1075

ISSUE TYPE

Docs Pull Request

COMPONENT NAME

ec2_customer_gateway
ADDITIONAL INFORMATION

While current documentation for the parameter states that
Border Gateway Protocol (BGP) Autonomous System Number (ASN), required when I(state=present).,
according to  boto3 documentation, bgp_asn defaults to 65000.

Reviewed-by: Markus Bergholz <git@osuv.de>
Reviewed-by: Alina Buzachis <None>
(cherry picked from commit 877ebbd)

plugins/modules/ec2_customer_gateway.py

@@ -23,7 +23,8 @@
 options:
   bgp_asn:
     description:
-      - Border Gateway Protocol (BGP) Autonomous System Number (ASN), required when I(state=present).
+      - Border Gateway Protocol (BGP) Autonomous System Number (ASN).
+      - Defaults to C(65000) if not specified when I(state=present).
     type: int
   ip_address:
     description: