[naga spv-in] Doc fix: Parser -> Frontend

Fix documentation left unchanged when `Parser` was renamed to
`Frontend`.

Author: jimblandy

naga/src/front/spv/mod.rs

@@ -313,14 +313,14 @@ struct LookupVariable {
     type_id: spirv::Word,
 }
 
-/// Information about SPIR-V result ids, stored in `Parser::lookup_expression`.
+/// Information about SPIR-V result ids, stored in `Frontend::lookup_expression`.
 #[derive(Clone, Debug)]
 struct LookupExpression {
     /// The `Expression` constructed for this result.
     ///
     /// Note that, while a SPIR-V result id can be used in any block dominated
     /// by its definition, a Naga `Expression` is only in scope for the rest of
-    /// its subtree. `Parser::get_expr_handle` takes care of spilling the result
+    /// its subtree. `Frontend::get_expr_handle` takes care of spilling the result
     /// to a `LocalVariable` which can then be used anywhere.
     handle: Handle<crate::Expression>,