Update Array.xml change ArgumentOutOfRangeException description for '…

…startIndex' parameter on FindIndex methods

Changed it to match String.Substring, because the behavior is identical (and original description is arguably wrong or hard to understand).

Fixes: dotnet#9719

xml/System/Array.xml

@@ -3772,7 +3772,7 @@
 
  <paramref name="match" /> is <see langword="null" />.</exception>
         <exception cref="T:System.ArgumentOutOfRangeException">
-          <paramref name="startIndex" /> is outside the range of valid indexes for <paramref name="array" />.</exception>
+          <paramref name="startIndex" /> is less than zero or greater than the length of the <paramref name="array" />.</exception>
         <altmember cref="M:System.Array.Exists``1(``0[],System.Predicate{``0})" />
         <altmember cref="M:System.Array.Find``1(``0[],System.Predicate{``0})" />
         <altmember cref="M:System.Array.FindLast``1(``0[],System.Predicate{``0})" />
@@ -3863,7 +3863,7 @@
 
  <paramref name="match" /> is <see langword="null" />.</exception>
         <exception cref="T:System.ArgumentOutOfRangeException">
-          <paramref name="startIndex" /> is outside the range of valid indexes for <paramref name="array" />.
+          <paramref name="startIndex" /> is less than zero or greater than the length of the <paramref name="array" />.
 
  -or-