Avoid allocating an array (#55109)

src/libraries/System.Security.Cryptography.X509Certificates/src/Internal/Cryptography/Pal.iOS/AppleCertificatePal.Pem.cs

@@ -14,7 +14,7 @@ namespace Internal.Cryptography.Pal
     internal sealed partial class AppleCertificatePal : ICertificatePal
     {
         // Byte representation of "-----BEGIN "
-        private static byte[] pemBegin = new byte[] { 0x2D, 0x2D, 0x2D, 0x2D, 0x2D, 0x42, 0x45, 0x47, 0x49, 0x4E, 0x20 };
+        private static ReadOnlySpan<byte> PemBegin => new byte[] { 0x2D, 0x2D, 0x2D, 0x2D, 0x2D, 0x42, 0x45, 0x47, 0x49, 0x4E, 0x20 };
 
         internal delegate bool DerCallback(ReadOnlySpan<byte> derData, X509ContentType contentType);
 
@@ -30,7 +30,7 @@ internal static bool TryDecodePem(ReadOnlySpan<byte> rawData, DerCallback derCal
             // Look for the PEM marker. This doesn't guarantee it will be a valid PEM since we don't check whether
             // the marker is at the beginning of line or whether the line is a complete marker. It's just a quick
             // check to avoid conversion from bytes to characters if the content is DER encoded.
-            if (rawData.IndexOf(pemBegin) < 0)
+            if (rawData.IndexOf(PemBegin) < 0)
             {
                 return false;
             }