Replace backticks with RDoc markup [ci-skip]

Follow-up to #50482.

RDoc does not support backticks the way that Markdown does.  Instead,
inline code must be wrapped with `+` or `<tt>`.

activerecord/lib/active_record/relation.rb

@@ -300,8 +300,8 @@ def find_or_initialize_by(attributes, &block)
     # Note that this method actually runs the queries, since the results of some
     # are needed by the next ones when eager loading is going on.
     #
-    # To run EXPLAIN on queries created by `first`, `pluck` and `count`, call
-    # these methods on `explain`:
+    # To run EXPLAIN on queries created by +first+, +pluck+ and +count+, call
+    # these methods on +explain+:
     #
     #   User.all.explain.count
     #   # EXPLAIN SELECT COUNT(*) FROM `users`