Documentation fixes
Documentation fixes

* Changed capitalization to `PerfKit` to match GitHub repository name.
* Fixed a few typos.
* Unwrapped a few commands so they can be copy-pasted
* Missing popd

GoogleCloudPlatform/PerfKitExplorer,GoogleCloudPlatform/PerfKitExplorer,GoogleCloudPlatform/PerfKitExplorer,dq922/PerfKitExplorer,dq922/PerfKitExplorer,dq922/PerfKitExplorer,dq922/PerfKitExplorer,GoogleCloudPlatform/PerfKitExplorer