Update README.md to indicate improved tempdisk capacity (14GB -> 74GB)

[joshuacwnewton]

This is a very useful action, and saved us a ton of headache!

From the README:

At the time of writing, public Github-hosted runners are using Azure DS2_v2 virtual machines, featuring a 84GB OS disk on / and a 14GB temp disk mounted on /mnt.

Today, when checking disk space before and after, I noticed the following instead:

# before
  66G   74G /mnt           /dev/sdb1
  21G   73G /              /dev/root
  99M  105M /boot/efi      /dev/sda15

# after
  87G   87G /home/runner/work/repo/repo /dev/mapper/buildvg-buildlv
 512M   73G /                           /dev/root
 100M   74G /mnt                        /dev/sdb1
  99M  105M /boot/efi                   /dev/sda15

87GB of free space, wow! It looks like whichever Azure VM they're using provides a much larger tempdisk now, which makes this action even more useful than it was before.