fix(proc-macros): align deprecated method stderr indentation

Match the single-space indentation used in other successful UI tests.

Author: soul022

proc-macros/tests/ui/incorrect/rpc/rpc_deprecated_method.stderr

@@ -1,12 +1,12 @@
 error: use of deprecated method `DeprecatedClient::async_method`: please use `new_method` instead
-  --> tests/ui/incorrect/rpc/rpc_deprecated_method.rs:63:20
-   |
+ --> tests/ui/incorrect/rpc/rpc_deprecated_method.rs:63:20
+  |
 63 |     assert_eq!(client.async_method().await.unwrap(), 16);
-   |                       ^^^^^^^^^^^^
-   |
+  |                       ^^^^^^^^^^^^
+  |
 note: the lint level is defined here
-  --> tests/ui/incorrect/rpc/rpc_deprecated_method.rs:4:9
-   |
-4  | #![deny(warnings)]
-   |         ^^^^^^^^
-   = note: `#[deny(deprecated)]` implied by `#[deny(warnings)]`
+ --> tests/ui/incorrect/rpc/rpc_deprecated_method.rs:4:9
+  |
+4 | #![deny(warnings)]
+  |         ^^^^^^^^
+  = note: `#[deny(deprecated)]` implied by `#[deny(warnings)]`