[MLIR][NFC] Update syntax of global_memref in ODS description.

[jurahul]

Summary

• The ODS description was using an old syntax that was updated during the review. This fixes the ODS description to match the current syntax.

mlir/include/mlir/Dialect/StandardOps/IR/Ops.td

let description = [{
     retrieve the memref for the global variable. Note that the memref
     for such global variable itself is immutable (i.e., get_global_memref for a
     given global variable will always return the same memref descriptor).
 
     Example:
 
     ```mlir
     // Private variable with an initial value.
-    global_memref @x : memref<2xf32> { sym_visibility = "private",
-                         initial_value = dense<0.0,2.0> : tensor<2xf32> }
+    global_memref "private" @x : memref<2xf32> = dense<0.0,2.0>
 
-    // External variable.
-    global_memref @y : memref<4xi32> { sym_visibility = "public" }
+    // Declaration of an external variable.
+    global_memref "private" @y : memref<4xi32>
 
     // Uninitialized externally visible variable.
-    global_memref @z : memref<3xf16> { sym_visibility = "public",
-                         initial_value }
+    global_memref @z : memref<3xf16> = uninitialized
+
+    // Externally visibile constant variable.
+    global_memref constant @c : memref<2xi32> = dense<1, 4>
     ```
   }];
 
   let arguments = (ins
       SymbolNameAttr:$sym_name,
       OptionalAttr<StrAttr>:$sym_visibility,
       TypeAttr:$type,
       OptionalAttr<AnyAttr>:$initial_value,