private/protocol/rest: Fix typo in build.go (#3907)

Cleans up documentation typo

private/protocol/rest/build.go

@@ -98,7 +98,7 @@ func buildLocationElements(r *request.Request, v reflect.Value, buildGETQuery bo
 
  // Support the ability to customize values to be marshaled as a
  // blob even though they were modeled as a string. Required for S3
- // API operations like SSECustomerKey is modeled as stirng but
+ // API operations like SSECustomerKey is modeled as string but
  // required to be base64 encoded in request.
  if field.Tag.Get("marshal-as") == "blob" {
  m = m.Convert(byteSliceType)