Fix Inconsistent definition of czstring in comments

Comments in sections  SL.str.3 and GSL.view disagree on whether czstring may be the nullptr.

This PR fixes the first comment definition in SL.str.3 that czstring `is a C-style string that is not the nullptr`

### SL.str.3: Use zstring or czstring to refer to a C-style, zero-terminated, sequence of characters
```
void f1(zstring s);     // s is a C-style string or the nullptr
void f1(czstring s);    // s is a C-style string that is not the nullptr
```
### GSL.view: Views
`zstring` // a `char*` supposed to be a C-style string; that is, a zero-terminated sequence of `char` or `nullptr`
`czstring` // a `const char*` supposed to be a C-style string; that is, a zero-terminated sequence of const `char` or `nullptr`

CppCoreGuidelines.md

@@ -18755,7 +18755,7 @@ Distinguishing these alternatives prevents misunderstandings and bugs.
 All we know is that it is supposed to be the nullptr or point to at least one character
 
     void f1(zstring s);     // s is a C-style string or the nullptr
-    void f1(czstring s);    // s is a C-style string that is not the nullptr
+    void f1(czstring s);    // s is a C-style string constant or the nullptr
     void f1(std::byte* s);  // s is a pointer to a byte (C++17)
 
 ##### Note