Docs: Fix typo in command to enable feature flag

What does this MR do?

The correct command to enable a feature flag is enable, not enabled:

[2] pry(main)> Feature.enabled(:anonymous_visual_review_feedback)
NoMethodError: undefined method `enabled' for Feature:Class
Did you mean?  enabled?
               enable
from (pry):2:in `<main>'
[3] pry(main)> Feature.enable(:anonymous_visual_review_feedback)
   (0.6ms)  BEGIN
  Feature::FlipperFeature Load (0.4ms)  SELECT  "features".* FROM "features" WHERE "features"."key" = $1 ORDER BY "features"."id" ASC LIMIT $2  [["key", "anonymous_visual_review_feedback"], ["LIMIT", 1]]
  Feature::FlipperFeature Create (2.1ms)  INSERT INTO "features" ("key", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["key", "anonymous_visual_review_feedback"], ["created_at", "2019-12-24 09:26:57.079143"], ["updated_at", "2019-12-24 09:26:57.079143"]]
   (3.2ms)  COMMIT
   (0.4ms)  BEGIN
  Feature::FlipperGate Load (0.7ms)  SELECT "feature_gates".* FROM "feature_gates" WHERE "feature_gates"."feature_key" = $1 AND "feature_gates"."key" = $2  [["feature_key", "anonymous_visual_review_feedback"], ["key", "boolean"]]
  Feature::FlipperGate Create (0.8ms)  INSERT INTO "feature_gates" ("feature_key", "key", "value", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["feature_key", "anonymous_visual_review_feedback"], ["key", "boolean"], ["value", "true"], ["created_at", "2019-12-24 09:26:57.105040"], ["updated_at", "2019-12-24 09:26:57.105040"]]
   (3.3ms)  COMMIT
=> nil

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