[Doc] Fix manual install & upgrade guide

[PeterDaveHello]

Ref: https://stackoverflow.com/a/7979255
Fix #1772

[joshsleeper]

I don't know enough about git describe and git rev-list to know if this is an appropriate use of these commands, but I do know that it seems to work as expected (assuming that's the expected response)

[USER@HOSTNAME .nvm]$ git checkout `git describe --abbrev=0 --tags --match "v[0-9]*" $(git rev-list --tags --max-count=1)`
HEAD is now at 7ad6d98... v0.33.8

---

more testing, this time manually checking out the v0.33.7 tag and then using the upgrade checkout command to verify it moved me to v0.33.8, which it did!

[USER@HOSTNAME .nvm]$ git checkout `git describe --abbrev=0 --tags --match "v0.33.7" $(git rev-list --tags --max-count=1)`
Previous HEAD position was 7ad6d98... v0.33.8
HEAD is now at 65f0572... v0.33.7
[USER@HOSTNAME .nvm]$ git checkout `git describe --abbrev=0 --tags --match "v[0-9]*" $(git rev-list --tags --max-count=1)`
Previous HEAD position was 65f0572... v0.33.7
HEAD is now at 7ad6d98... v0.33.8