Fix typo in metadata (dotnet#4461)

Author: GeertvanHorrik

xml/System.Reflection.Metadata/PEReaderExtensions.xml

@@ -58,7 +58,7 @@
       <Docs>
         <param name="peReader">The current <see cref="T:System.Reflection.PortableExecutable.PEReader" /> instance.</param>
         <summary>Gets a <see cref="T:System.Reflection.Metadata.MetadataReader" /> from a <see cref="T:System.Reflection.PortableExecutable.PEReader" />.</summary>
-        <returns>A metdata reader.</returns>
+        <returns>A metadata reader.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[