Fixed documentation for custom locator example, so argument lines up with usage (#1175)

* Fixed documentation for custom locator example, so argument lines up with usage.

Author: DanielPBak

src/SeleniumLibrary/__init__.py

@@ -195,13 +195,13 @@ class SeleniumLibrary(DynamicCore):
     locators is a two part process. First, create a keyword that returns
     a WebElement that should be acted on:
 
-    | Custom Locator Strategy | [Arguments] | ${browser} | ${strategy} | ${tag} | ${constraints} |
-    |   | ${element}= | Execute Javascript | return window.document.getElementById('${criteria}'); |
+    | Custom Locator Strategy | [Arguments] | ${browser} | ${locator} | ${tag} | ${constraints} |
+    |   | ${element}= | Execute Javascript | return window.document.getElementById('${locator}'); |
     |   | [Return] | ${element} |
 
     This keyword is a reimplementation of the basic functionality of the
     ``id`` locator where ``${browser}`` is a reference to a WebDriver
-    instance and ``${strategy}`` is name of the locator strategy. To use
+    instance and ``${locator}`` is name of the locator strategy. To use
     this locator it must first be registered by using the
     `Add Location Strategy` keyword: