Two minor changes: make Checksums a caseless enum, fix a grammar mistake in a code comment

Author: tsolomko

Sources/Common/CheckSums.swift

@@ -5,7 +5,7 @@
 
 import Foundation
 
-struct CheckSums {
+enum CheckSums {
 
     // MARK: Functions
 

Sources/ZIP/LittleEndianByteReader+Zip.swift

@@ -29,7 +29,7 @@ fileprivate extension String {
 
     // "Latin-US (DOS)" CP437-2147483120
     // Note that this encoding is interpreted differently "depending on the context". This concerns codes in 0-31 range
-    // and code 127: sometimes they treated as normal characters (as was defined by IBM) and sometimes as control
+    // and code 127: sometimes they are treated as normal characters (as was defined by IBM) and sometimes as control
     // characters, matching corresponding ranges of ASCII/UTF-8. The second interpretation, however, doesn't make CP437
     // compatible with UTF-8, as CP437 contains codes in the 80-FF range which are not valid UTF-8 codes.
     // In any case, we are constrained to the implementation provided by Foundation, which currently treats them as