NFC: update CLanguageStandard.c89 docs in PackageDescription (#8320)

### Motivation:

Update typo in `CLanguageStandard` description.

### Modifications:

- Update description for `c89` standard to use correct year, 1989,
instead of 1990.

### Result:

- Documentation is improved!

Author: esnssr

Sources/PackageDescription/LanguageStandardSettings.swift

@@ -14,7 +14,7 @@
 /// package.
 public enum CLanguageStandard: String {
 
-    /// The identifier for the ISO C 1990 language standard.
+    /// The identifier for the ISO C 1989 language standard.
     case c89
 
     /// The identifier for the ISO C 1990 language standard.