Fix typo in Span<T>.xml (dotnet#5323)

The following sentence:

> ...methods of the `Span<T>` **class**...

Uses the word `class` to refer to the `Span<T>` type which is in fact a (ref) `struct`. I have changed it to read as follows:

> ...methods of the `Span<T>` **type**...

Author: ericmutta

xml/System/Span`1.xml

@@ -70,7 +70,7 @@ The following example uses the C# [stackalloc](/dotnet/csharp/language-reference
 
 [!code-csharp[Creating a Span\<Byte> from stack-allocated memory](~/samples/snippets/csharp/api/system/span/program.cs#3)]
 
-Because `Span<T>` is an abstraction over an arbitrary block of memory, methods of the `Span<T>` class and methods with `Span<T>` parameters operate on any `Span<T>` object regardless of the kind of memory it encapsulates. For example, each of the separate sections of code that initialize the span and calculate the sum of its elements can be changed into single initialization and calculation methods, as the following example illustrates:
+Because `Span<T>` is an abstraction over an arbitrary block of memory, methods of the `Span<T>` type and methods with `Span<T>` parameters operate on any `Span<T>` object regardless of the kind of memory it encapsulates. For example, each of the separate sections of code that initialize the span and calculate the sum of its elements can be changed into single initialization and calculation methods, as the following example illustrates:
 
 [!code-csharp[Creating a Span\<Byte> from stack-allocated memory](~/samples/snippets/csharp/api/system/span/program.cs#4)]