This is cosmetic change, it does not change behavior at all.
It was detected by the following rubocop configuration:

```
  Performance/StringIdentifierArgument:
    Enabled: true
```

Apparently, string identifier argument should not be used.

Benchmark result:

```
  ruby 3.2.8 (2025-03-26 revision 13f495dc2c) +YJIT [x86_64-linux]
  Warming up --------------------------------------
  check method with string notation
                         875.323k i/100ms
  check method with symbol notation
                           1.880M i/100ms
  Calculating -------------------------------------
  check method with string notation
                            8.828M (± 1.4%) i/s  (113.28 ns/i) -     44.641M in   5.058002s
  check method with symbol notation
                           19.766M (± 2.2%) i/s   (50.59 ns/i) -     99.631M in   5.043071s

  Comparison:
  check method with symbol notation: 19765521.3 i/s
  check method with string notation:  8827598.1 i/s - 2.24x  slower
```

Appendix: benchmark script

```ruby
require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips'
  gem 'benchmark-memory'
end

Benchmark.ips do |x|
  x.report("check method with string notation") {
    "test".respond_to?("start_with?")
  }
  x.report("check method with symbol notation") {
    "test".respond_to?(:start_with?)
  }
  x.compare!
end
```

Signed-off-by: Kentaro Hayashi <[email protected]>