mime.types.README: Clarify upstream version.

Clarify that the quoted commit SHA corresponds to version debian/3.61

Bug: 135431432
Test: Manually checked that the latest commit under
      https://salsa.debian.org/debian/mime-support/tree/debian/3.61
      has SHA d4bbcca4ba04582ad1d253d82fc139bb23841a43.

Change-Id: I66ad88bb07b6727f491ab3800ad84731ffc860b5
1 file changed