Fixes selectionPattern documentation

I can confirm that the the extra `\` was actually not valid and was causing my response to default to the last template in the list.

Author: avoliva

README.md

@@ -219,7 +219,7 @@ custom:
         cors: true
         response:
           - statusCode: 200
-            selectionPattern: '2\\d{2}'
+            selectionPattern: '2\d{2}'
             responseParameters: {}
             responseTemplates:
               application/json: |-
@@ -637,7 +637,7 @@ custom:
         cors: true
         response:
           - statusCode: 200
-            selectionPattern: '2\\d{2}'
+            selectionPattern: '2\d{2}'
             responseParameters: {}
             responseTemplates:
               application/json: |-
@@ -1009,4 +1009,4 @@ custom:
                 "success": false,
                 "errorMessage": "Client Error"
               }
-```
+```