Use a classic doubling of capacity rather than only adding
twice as much capacity as is already known to be needed.

```
compare-ruby: ruby 3.4.0dev (2024-09-04T09:21:53Z opt-strftime-2 ae98d19cf9) +YJIT [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-09-04T11:46:02Z opt-strftime-growth 586263d6fb) +YJIT [arm64-darwin23]
warming up...

|                            |compare-ruby|built-ruby|
|:---------------------------|-----------:|---------:|
|time.strftime("%FT%T")      |      1.754M|    1.889M|
|                            |           -|     1.08x|
|time.strftime("%FT%T.%3N")  |      1.508M|    1.749M|
|                            |           -|     1.16x|
|time.strftime("%FT%T.%6N")  |      1.488M|    1.756M|
|                            |           -|     1.18x|
compare-ruby: ruby 3.4.0dev (2024-09-04T09:21:53Z opt-strftime-2 ae98d19cf9) +YJIT [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-09-04T09:21:53Z opt-strftime-2 ae98d19cf9) +YJIT [arm64-darwin23]
warming up...
```