docs: fix 6 typos in documentation comments

[Avicennasis]

Summary

Fixes 6 spelling typos in documentation comments in PHManager.swift and
PHCache.swift. No identifiers, code, or behaviour are changed - every hunk
is inside a /// or /** */ doc comment.

• PHImageKit/PHManager.swift: cancel ongoing reques -> cancel ongoing requests;
  Convinience method -> Convenience method (x2); Purche memory and file cache
  -> Purge memory and file cache
• PHImageKit/PHCache.swift: Clear expired and overexeeded max size file cache
  -> Clear expired and exceeded max size file cache;
  while file chache is 50% -> while file cache is 50%

Deliberately left alone

Several other misspellings in this library are part of the public API surface
and cannot be corrected without a breaking change, so they are untouched:

• comletion: - the external parameter label of PHManager.imageData(_:comletion:).
  The - parameter comletion: doc tag correctly matches it.
• clearFileChache(_:) on PHCache, called from two places in PHManager.
• contentLenght: - the parameter label of PHDownload.progress(_:contentLenght:).
• arkColor - a property of PHProgressView.

The double space in DispatchQueue(label: imageKitDomain + ".ioQueue")
(PHFileCache.swift) is formatting rather than a typo and is also left as is.