Update Mathesar version within docs

[seancolsen]

This fixes a mistake I made during the 0.1.5 release process — I failed to update the version number within our docs. Without this fix, readers following our "from scratch" docs are likely to install 0.1.4 instead of 0.1.5.

I've added this step to our release process now.

Checklist

• [x] My pull request has a descriptive title (not a vague title like Update index.md).
• [x] My pull request targets the develop branch of the repository
• [x] My commit messages follow best practices.
• [x] My code follows the established code style of the repository.
• [x] I added tests for the changes I made (if applicable).
• [x] I added or updated documentation (if applicable).
• [x] I tried running the project locally and verified that there are no visible errors.

Developer Certificate of Origin

Developer Certificate of Origin

``` Developer Certificate of Origin Version 1.1 Copyright (C) 2004, 2006 The Linux Foundation and its contributors. 1 Letterman Drive Suite D4700 San Francisco, CA, 94129 Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Developer's Certificate of Origin 1.1 By making a contribution to this project, I certify that: (a) The contribution was created in whole or in part by me and I have the right to submit it under the open source license indicated in the file; or (b) The contribution is based upon previous work that, to the best of my knowledge, is covered under an appropriate open source license and I have the right under that license to submit that work with modifications, whether created in whole or in part by me, under the same open source license (unless I am permitted to submit under a different license), as indicated in the file; or (c) The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it. (d) I understand and agree that this project and the contribution are public and that a record of the contribution (including all personal information I submit with it, including my sign-off) is maintained indefinitely and may be redistributed consistent with this project or the open source license(s) involved. ```

[seancolsen]

I'm merging this without review since it's important and docs-only.

[kgodey]

Good catch @seancolsen