Fix incredibly minimal typo on namespace.rb documentation

[yurighensev]

Fix incredibly minimal typo on namespace.rb documentation.

[grape-bot]

	2 Warnings
⚠️	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#1689](https://github.com/ruby-grape/grape/pull/1689): Fix incredibly minimal typo on namespace.rb documentation - [@yurighensev](https://github.com/yurighensev).

Generated by 🚫 danger

[dblock]

Thanks!