Fix CustomerTools example to use existing methods

[filiphr]

The ToolContext does not have a get(String) method. This PR is fixing the documentation to show the methods that can be used.

However, I think that it would be better if ToolContext had a get(String) method that would look like:

public Object get(String key) {
    return this.context.get(key);
}

It could also be made generic:

public <T> T get(String key) {
    return (T) this.context.get(key);
}

If you prefer to add the method, I can go ahead and create a PR for that instead

[ilayaperumalg]

@filiphr Thanks for fixing the documentation to use the right method for ToolContext!

[ilayaperumalg]

Back-ported into 1.0.x as cd364e3