Address an upcoming violation of markdownlint rule MD050/strong-style.

This file (and only this file in the repo) uses both `__` and `**` to create a strong emphasis. This change makes the file consistent by changing the single instance of `__` to `**`. The current version of `markdownlint` and an upcoming version of `markdownlint-cli2` do/will flag this violation. The new rule can be disabled, but resolving it in this case is trivial.

More info: https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md050

docs/rules/array-callback-return.md

@@ -13,7 +13,7 @@ var indexMap = myArray.reduce(function(memo, item, index) {
 ## Rule Details
 
 This rule enforces usage of `return` statement in callbacks of array's methods.
-Additionally, it may also enforce the `forEach` array method callback to __not__ return a value by using the `checkForEach` option.
+Additionally, it may also enforce the `forEach` array method callback to **not** return a value by using the `checkForEach` option.
 
 This rule finds callback functions of the following methods, then checks usage of `return` statement.