Update testing docs to reflect Gradle wrapper

This commit updates the testing docs to reflect the usage of the Gradle
wrapper.

Relates #28107

Author: jasontedor

GRADLE.CHEATSHEET

@@ -1,4 +1,6 @@
-As a quick helper, below are the equivalent commands from maven to gradle (TESTING.md has also been updated). You can also run "gradle tasks" to see all tasks that are available to run.
+As a quick helper, below are the equivalent commands from Maven to Gradle
+(TESTING.md has also been updated). You can also run `./gradlew tasks` to see
+all tasks that are available to run.
 clean -> clean
 test -> test
 verify -> check

README.textile

@@ -202,9 +202,9 @@ We have just covered a very small portion of what Elasticsearch is all about. Fo
 
 h3. Building from Source
 
-Elasticsearch uses "Gradle":https://gradle.org for its build system. You'll need to have at least version 3.3 of Gradle installed.
+Elasticsearch uses "Gradle":https://gradle.org for its build system.
 
-In order to create a distribution, simply run the @gradle assemble@ command in the cloned directory.
+In order to create a distribution, simply run the @./gradlew assemble@ command in the cloned directory.
 
 The distribution for each project will be created under the @build/distributions@ directory in that project.