Fix pluralization mismatch on compile_time_binding/s (#6696)

Assisted-by: Google Antigravity with Gemini 3 Flash

toolchain/check/generic.cpp

@@ -424,7 +424,7 @@ auto DiscardGenericDecl(Context& context) -> void {
 
 auto BuildGeneric(Context& context, SemIR::InstId decl_id) -> SemIR::GenericId {
   auto all_bindings =
-      context.scope_stack().compile_time_bindings_stack().PeekAllValues();
+      context.scope_stack().compile_time_binding_stack().PeekAllValues();
 
   if (all_bindings.empty()) {
     CARBON_CHECK(context.generic_region_stack().PeekEvalBlock().empty(),

toolchain/check/scope_stack.h

@@ -183,7 +183,7 @@ class ScopeStack {
     return destroy_id_stack_;
   }
 
-  auto compile_time_bindings_stack() -> ArrayStack<SemIR::InstId>& {
+  auto compile_time_binding_stack() -> ArrayStack<SemIR::InstId>& {
     return compile_time_binding_stack_;
   }