[Tim from Marketing] Some minor doc changes for clarity.

[Naeddyr]

Just a typo:
Accessing a member of a variable which value is null will compile fine, but result in a NullReferenceException being thrown at runtime:

Should probably be something like

Accessing a member of a variable whose value is null will compile fine, but result in a NullReferenceException being thrown at runtime:

But as a beginner the explanation for the ?? operator is confusing:

csharp/exercises/concept/tim-from-marketing/.docs/introduction.md

Lines 62 to 68 in 7e614a2

	```csharp
	string? name1 = "John";
	name1 ?? "Paul"; // => "John"
	string? name2 = null;
	name2 ?? "George"; // => "George"
	```

This makes it seem like ?? isn't an operator (yes, I know it says in plain text that it's an operator) but a statement pseudo-assignment thing that marks name1 as having a default value to replace null. This also doesn't mention ??=. I noticed this when I was confused why the LSP was complaining about the statement not being valid. This just needs to be expanded a bit.

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