CPP Quickstart Setup directory variable name

[karlji]

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Issue description CPP quickstart guide contains wrongly named variable in setup part for Windows.

Steps to reproduce the issue

1. Go to 'https://grpc.io/docs/languages/cpp/quickstart/'
2. Scroll down to 'Setup' - 'Windows'
3. See error mkdir %INSTALL_DIR% instead of mkdir %MY_INSTALL_DIR%

Expected behavior Installation directory variable name matches with the one defined in previous steps %MY_INSTALL_DIR%

Screenshots

[Sangamesh1997]

@sreenithi Can you please look into this

[shivaspeaks]

Fixed in https://github.com/grpc/grpc.io/pull/1420