fix typo that prevents cocoapods installation

[luicm]

With Cocoapods version 1.8.3, try to update or install pod was sending this error messages:

- `Timepiece-library` does not specify a Swift version and none of the targets (`Pods`) integrating it have the `SWIFT_VERSION` attribute set. Please contact the author or set the `SWIFT_VERSION` attribute in at least one of the targets that integrate this pod.

after some investigation, there was a small typo in the podspect. Fixed the typo there is no error anymore