[Typo] v15.2.x/UPGRADING.md

[fawby-republic]

Problem

Typo here:

https://github.com/cosmos/gaia/blob/release/v15.2.x/UPGRADING.md#follow-these-steps-if-gaiad-repo-already-present

   cd $HOME/.gaia
   git pull
   git fetch --tags
   git checkout v15.2.0
   make install

The . in the line cd $HOME/.gaia is a typo.

Closing criteria

The . in the line cd $HOME/.gaia should be removed, and if this document is derived from a template, the template should be corrected as well to prevent further propagation.

Problem details

.gaia is the name of the default chain directory, and not the repo.
This typo slowed down a few validators this upgrade, and it was also raised that typo has existed in prior upgrade documentation.