Fix underscore

[dimaush]

Несущественные исправления без существенных изменений тестов или кода программы.

[vtm9]

Moving this task to its first-tag directory is appropriate, but the solution rewrite changes existing behavior after digits by dropping \d from the lowercase-to-uppercase boundary regex.

Reproduction: solution("version2Thing") changes from "version2_thing" to "version2thing"; "HTML5Parser" changes from "html5_parser" to "html5parser". These inputs satisfy the published limits.

Please retain the digit boundary, e.g. ([a-z\d])([A-Z]), and add regression asserts. All 25 existing asserts pass but do not exercise this case.