Vb.net: Fix typo in enum member syntax description (#49666)

- Was [<attribute list>] member name [ = initializer ],
- Should be [<attribute list>] membername [ = initializer ],
- Reason: `membername` is a single identifier; later in the document it is referred to as `membername`

Author: maciejpirog

docs/visual-basic/language-reference/statements/enum-statement.md

@@ -66,7 +66,7 @@ End Enum
 
   Required. List of member constants being declared in this statement. Multiple members appear on individual source code lines.
 
-  Each `member` has the following syntax and parts: `[<attribute list>] member name [ = initializer ]`
+  Each `member` has the following syntax and parts: `[<attribute list>] membername [ = initializer ]`
 
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