[mpp, tests] Minor: fix incorrect testdata

It was missed on rr/CI because those tests run only on macs, and
currrently CI has some troubles with that

libraries/tools/kotlin-gradle-plugin/src/functionalTest/resources/expectedDiagnostics/KotlinDefaultHierarchyFallbackNativeTargetShortcutUsageDetected-ios.txt

@@ -14,7 +14,7 @@ Please declare the required targets explicitly:
       /* ... */
   }
 
-After that, replace `by getting` with a static accessor:
+After that, replace `by getting` with static accessors:
 
   sourceSets {
       commonMain { ... }

libraries/tools/kotlin-gradle-plugin/src/functionalTest/resources/expectedDiagnostics/KotlinDefaultHierarchyFallbackNativeTargetShortcutUsageDetected-tvos.txt

@@ -14,7 +14,7 @@ Please declare the required targets explicitly:
       /* ... */
   }
 
-After that, replace `by getting` with a static accessor:
+After that, replace `by getting` with static accessors:
 
   sourceSets {
       commonMain { ... }

libraries/tools/kotlin-gradle-plugin/src/functionalTest/resources/expectedDiagnostics/KotlinDefaultHierarchyFallbackNativeTargetShortcutUsageDetected-watchos.txt

@@ -14,7 +14,7 @@ Please declare the required targets explicitly:
       /* ... */
   }
 
-After that, replace `by getting` with a static accessor:
+After that, replace `by getting` with static accessors:
 
   sourceSets {
       commonMain { ... }