docs: fix text_splitters mdx code example never closed

[x] Read the Contributing documentation.
[x] Read the Code of conduct documentation.
[x] Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
[x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
[x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
[x] Describes the source of new concepts.
[x] References existing implementations as appropriate.
[x] Contains test coverage for new functions.
[ ] Passes all golangci-lint checks. (really needed for this case ?)

tmc / langchaingo