Update Binder target to microsoft:main for GDPCIR dataset

[delgadom]

Currently the binder button clones the ClimateImpactLab/PlanetaryComputerExamples fork. This updates to point to the mcirosoft repo's main branch. I just tested this by navigating to the URL and the updated link works correctly.

Parsing the components of the link:

# "launch binder" image
[![Binder](https://mybinder.org/badge_logo.svg)](
    # binder env is a repo we put together with the env alone to minimize build time
    https://mybinder.org/v2/gh/ClimateImpactLab/downscaleCMIP6-binder-env/main?
       # specify git-pull on open
       urlpath=git-pull%3F
            # pull the microsoft/PlanetaryComputerExamples repo
            repo%3Dhttps%253A%252F%252Fgithub.com%252Fmicrosoft%252FPlanetaryComputerExamples%26
            # open jupyterlab
            urlpath%3Dlab%252F
            # open at PlanetaryComputerExamples/datasets/cil-gdpcir/README.md
            tree%252FPlanetaryComputerExamples%252Fdatasets%252Fcil-gdpcir%252FREADME.md%26
            # on branch: main
            branch%3Dmain
)

I put this together with the nbgitpuller tool: https://jupyterhub.github.io/nbgitpuller/link?tab=binder

[TomAugspurger]

Thanks!