Fix ToHexString typos (#4544)

Changes hecxadecimal to hexadecimal.

Author: swilliams96

xml/System.Diagnostics/ActivitySpanId.xml

@@ -342,7 +342,7 @@ The characters in <paramref name="idData" /> are not all lower-case hexadecimal
       <Parameters />
       <Docs>
         <summary>Returns a 16-character hexadecimal string that represents this span ID.</summary>
-        <returns>The 16-character hecxadecimal string representation of this span ID.</returns>
+        <returns>The 16-character hexadecimal string representation of this span ID.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>

xml/System.Diagnostics/ActivityTraceId.xml

@@ -368,7 +368,7 @@ The characters in <paramref name="idData" /> are not all lower-case hexadecimal
       <Parameters />
       <Docs>
         <summary>Returns a 32-character hexadecimal string that represents this trace ID.</summary>
-        <returns>The 32-character hecxadecimal string representation of this trace ID.</returns>
+        <returns>The 32-character hexadecimal string representation of this trace ID.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>