Fix comment about function variables

Change-Id: Ic639b8f93140363ce273d06b6b052658ea2b1c27
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/18705
Reviewed-by: dan sinclair <dsinclair@google.com>

src/ast/variable.h

@@ -43,7 +43,7 @@ class DecoratedVariable;
 ///       var<private> height : i32 = 3;  // with initializer
 ///
 ///       // A variable declared inside a function doesn't take a storage class,
-///       // and maps to SPIR-V Private storage.
+///       // and maps to SPIR-V Function storage.
 ///       var computed_depth : i32;
 ///       var area : i32 = compute_area(width, height);
 ///