minor doc fixes to get_url

lib/ansible/modules/network/basics/get_url.py

@@ -46,8 +46,6 @@ options:
     description:
       - HTTP, HTTPS, or FTP URL in the form (http|https|ftp)://[user[:pass]]@host.domain[:port]/path
     required: true
-    default: null
-    aliases: []
   dest:
     description:
       - absolute path of where to download the file to.
@@ -57,7 +55,6 @@ options:
         If C(dest) is a directory, the file will always be
         downloaded (regardless of the force option), but replaced only if the contents changed.
     required: true
-    default: null
   force:
     description:
       - If C(yes) and C(dest) is not a directory, will download the file every
@@ -100,7 +97,7 @@ options:
     version_added: '1.8'
   headers:
     description:
-       - Add custom HTTP headers to a request in the format 'key:value,key:value'
+        - 'Add custom HTTP headers to a request in the format "key:value,key:value"'
     required: false
     default: null
     version_added: '2.0'