Fix spelling mistakes in d/changelog
lintian complains:
W: initramfs-tools: spelling-error-in-changelog depency dependency [usr/share/doc/initramfs-tools/changelog.gz]
W: initramfs-tools: spelling-error-in-changelog plaforms platforms [usr/share/doc/initramfs-tools/changelog.gz]
W: initramfs-tools-core: spelling-error-in-changelog depency dependency [usr/share/doc/initramfs-tools-core/changelog.gz]
W: initramfs-tools-core: spelling-error-in-changelog plaforms platforms [usr/share/doc/initramfs-tools-core/changelog.gz]