Replace Identifier misspelling in license headers

[ottodevs]

Even though it was fun to spot projects reusing or forking aragon contracts: https://github.com/search?q=SPDX-License-Identitifer&type=Code

But I thought it might be interesting to fix the misspelling, in case any automated script might eventually parse that header to show the license.

Related: aragon/aragonOS#558

[coveralls]

Coverage increased (+0.2%) to 97.763% when pulling 5d882a1 on ottodevs:identitifer-fix into e90e917 on aragon:master.

[sohkai]

Thanks @ottodevs! I've just rebased this change against next since that's where any new contract changes go :).