Categories in "Accessibility" are incorrect (#6207)

* Categories in "Accessibility" are incorrect

-The brief provided for "private protected" should be the brief for "protected internal" instead. 
-As per my knowledge in c#, "private protected" access modifier does not exist. If it does what does this exactly do?

* Update classes-and-objects.md

-Updated brief for protected internal as per the review comment
-Added private protected back with some modification as per the reference doc.

* Update classes-and-objects.md

Added back the private protected with its original brief.

* Updated the number of categories of accessiblility

Updated the number of categories of accessiblility from five to six.

* quick grammar fix

Author: aditimantri2196

docs/csharp/tour-of-csharp/classes-and-objects.md

@@ -49,7 +49,7 @@ The following provides an overview of the kinds of members a class can contain.
 
 ## Accessibility
 
-Each member of a class has an associated accessibility, which controls the regions of program text that are able to access the member. There are five possible forms of accessibility. These are summarized below.
+Each member of a class has an associated accessibility, which controls the regions of program text that are able to access the member. There are six possible forms of accessibility. These are summarized below.
 
 * `public`
 	- Access not limited
@@ -58,11 +58,11 @@ Each member of a class has an associated accessibility, which controls the regio
 * `internal`
 	- Access limited to the current assembly (.exe, .dll, etc.)
 * `protected internal`
-	- Access limited to the containing class or classes derived from the containing class
+	- Access limited to the containing class, classes derived from the containing class, or classes within the same assembly
 * `private`
 	- Access limited to this class
 * `private protected`
-    - Access limited to the containing class or classes derived from the containing type within the same assembly
+	- Access limited to the containing class or classes derived from the containing type within the same assembly
 
 ## Type parameters