Apply suggestions from code review

Use ticks around abis.

Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

compiler/rustc_hir_analysis/src/lib.rs

@@ -119,10 +119,10 @@ use bounds::Bounds;
 
 fn require_c_abi_if_c_variadic(tcx: TyCtxt<'_>, decl: &hir::FnDecl<'_>, abi: Abi, span: Span) {
     const ERROR_HEAD: &str = "C-variadic function must have a compatible calling convention";
-    const CONVENTIONS_UNSTABLE: &str = "C, cdecl, win64, sysv64 or efiapi";
-    const CONVENTIONS_STABLE: &str = "C or cdecl";
+    const CONVENTIONS_UNSTABLE: &str = "`C`, `cdecl`, `win64`, `sysv64` or `efiapi`";
+    const CONVENTIONS_STABLE: &str = "`C` or `cdecl`";
     const UNSTABLE_EXPLAIN: &str =
-        "using different calling convention than C or cdecl for varargs functions is unstable";
+        "using different calling convention than `C` or `cdecl` for varargs functions is unstable";
 
     if !decl.c_variadic || matches!(abi, Abi::C { .. } | Abi::Cdecl { .. }) {
         return;