fix(types): BCardSubTitle component declared export name

[mblonyox]

When we importing b-card-sub-title component individually, typescript throws following error:

Module '"../../node_modules/bootstrap-vue/src"' has no exported member 'BCardSubTitle'. Did you mean 'BCardSubtitle'?

Describe the PR

Fix incorrect export declaration of BCardSubTitle component.

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Enhancement
• ARIA accessibility
• Documentation update
• Other (please describe)

Does this PR introduce a breaking change? (check one)

• Yes (please describe)
• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (i.e. [...] (fixes #xxx[,#xxx]), where "xxx" is the issue number)
• It's submitted to the dev branch, not the master branch
• It should address only one issue or feature. If adding multiple features or fixing a bug and adding a new feature, break them into separate PRs if at all possible.
• The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, fix typos, chore: fix typo in README, etc). This is very important, as the CHANGELOG is generated from these messages.

If new features/enhancement/fixes are added or changed:

• Includes documentation updates (including updating the component's package.json for slot and event changes)
• Includes any needed TypeScript declaration file updates
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (Does it affect screen reader users or keyboard only users? Clickable items should be in the tab index, etc.)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

[codecov]

Codecov Report

Merging #4229 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #4229   +/-   ##
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  Coverage   99.86%   99.86%           
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  Files         237      237           
  Lines        4525     4525           
  Branches     1274     1274           
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  Hits         4519     4519           
  Misses          5        5           
  Partials        1        1

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[tmorehouse]

Thanks!