Update Colors.md, removed duplicate colors

Summary:
removed duplicate colors

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Closes https://github.com/facebook/react-native/pull/13555

Differential Revision: D4908510

Pulled By: javache

fbshipit-source-id: 819a3c2788232bc092cccc071f115853e7caddad

docs/Colors.md

@@ -76,7 +76,6 @@ You can also use color names as values. React Native follows the [CSS3 specifica
 - <color darksalmon /> darksalmon (#e9967a)
 - <color darkseagreen /> darkseagreen (#8fbc8f)
 - <color darkslateblue /> darkslateblue (#483d8b)
-- <color darkslategray /> darkslategray (#2f4f4f)
 - <color darkslategrey /> darkslategrey (#2f4f4f)
 - <color darkturquoise /> darkturquoise (#00ced1)
 - <color darkviolet /> darkviolet (#9400d3)
@@ -118,7 +117,6 @@ You can also use color names as values. React Native follows the [CSS3 specifica
 - <color lightsalmon /> lightsalmon (#ffa07a)
 - <color lightseagreen /> lightseagreen (#20b2aa)
 - <color lightskyblue /> lightskyblue (#87cefa)
-- <color lightslategray /> lightslategray (#778899)
 - <color lightslategrey /> lightslategrey (#778899)
 - <color lightsteelblue /> lightsteelblue (#b0c4de)
 - <color lightyellow /> lightyellow (#ffffe0)
@@ -173,7 +171,6 @@ You can also use color names as values. React Native follows the [CSS3 specifica
 - <color skyblue /> skyblue (#87ceeb)
 - <color slateblue /> slateblue (#6a5acd)
 - <color slategray /> slategray (#708090)
-- <color slategrey /> slategrey (#708090)
 - <color snow /> snow (#fffafa)
 - <color springgreen /> springgreen (#00ff7f)
 - <color steelblue /> steelblue (#4682b4)