[clang] Revise the modules document for clarity (PR #90237)

[Erich Keane via cfe-commits]

================
@@ -289,20 +277,20 @@ we can't compile them by the original command lines. But we are still able to do
   $ ./Hello.out
   Hello World!
 
-Module name requirement
-~~~~~~~~~~~~~~~~~~~~~~~
+Module name requirements
+~~~~~~~~~~~~~~~~~~~~~~~~
 
-[module.unit]p1 says:
+..
 
-.. code-block:: text
+  [module.unit]p1:
 
   All module-names either beginning with an identifier consisting of std followed by zero
   or more digits or containing a reserved identifier ([lex.name]) are reserved and shall not
   be specified in a module-declaration; no diagnostic is required. If any identifier in a reserved
   module-name is a reserved identifier, the module name is reserved for use by C++ implementations;
   otherwise it is reserved for future standardization.
 
-So all of the following name is not valid by default:
+So none of the following names are valid by default:
----------------
erichkeane wrote:

```suggestion
Therefore, none of the following names are valid by default:
```

https://github.com/llvm/llvm-project/pull/90237