docs($httpProvider): fix description of useLegacyPromiseExtensions

useLegacyPromiseExtensions's default value is true, and the  legacy
methods exist when it is set to true.

Closes angular#12974

Author: johnz133

src/ng/http.js

@@ -345,9 +345,9 @@ function $HttpProvider() {
    * Configure `$http` service to return promises without the shorthand methods `success` and `error`.
    * This should be used to make sure that applications work without these methods.
    *
-   * Defaults to false. If no value is specified, returns the current configured value.
+   * Defaults to true. If no value is specified, returns the current configured value.
    *
-   * @param {boolean=} value If true, `$http` will return a normal promise without the `success` and `error` methods.
+   * @param {boolean=} value If true, `$http` will return a promise with the deprecated legacy `success` and `error` methods.
    *
    * @returns {boolean|Object} If a value is specified, returns the $httpProvider for chaining.
    *    otherwise, returns the current configured value.