Fix link and description of supported Markdown (#1490)

See: http://facebook.github.io/graphql/June2018/#sec--deprecated

Author: IvanGoncharov

src/type/directives.js

@@ -153,8 +153,8 @@ export const GraphQLDeprecatedDirective = new GraphQLDirective({
       type: GraphQLString,
       description:
         'Explains why this element was deprecated, usually also including a ' +
-        'suggestion for how to access supported similar data. Formatted ' +
-        'in [Markdown](https://daringfireball.net/projects/markdown/).',
+        'suggestion for how to access supported similar data. Formatted using ' +
+        'the Markdown syntax (as specified by [CommonMark](https://commonmark.org/).',
       defaultValue: DEFAULT_DEPRECATION_REASON,
     },
   },

src/utilities/__tests__/schemaPrinter-test.js

@@ -629,8 +629,8 @@ describe('Type System Printer', () => {
       directive @deprecated(
         """
         Explains why this element was deprecated, usually also including a suggestion
-        for how to access supported similar data. Formatted in
-        [Markdown](https://daringfireball.net/projects/markdown/).
+        for how to access supported similar data. Formatted using the Markdown syntax
+        (as specified by [CommonMark](https://commonmark.org/).
         """
         reason: String = "No longer supported"
       ) on FIELD_DEFINITION | ENUM_VALUE
@@ -865,8 +865,8 @@ describe('Type System Printer', () => {
       # Marks an element of a GraphQL schema as no longer supported.
       directive @deprecated(
         # Explains why this element was deprecated, usually also including a suggestion
-        # for how to access supported similar data. Formatted in
-        # [Markdown](https://daringfireball.net/projects/markdown/).
+        # for how to access supported similar data. Formatted using the Markdown syntax
+        # (as specified by [CommonMark](https://commonmark.org/).
         reason: String = "No longer supported"
       ) on FIELD_DEFINITION | ENUM_VALUE