Fix typo in spec let

[#46491831]

The root element was web_page in the source for example that tests that
import_msf_web_vuln_element creates an Mdm::WebVuln.  The root element
name did not actually matter for the example, but it looked like an
error and was confusing to read the setup that root element was web_page
instead of the correct web_vuln.

spec/support/shared/examples/msf/db_manager/import_msf_xml.rb

@@ -967,7 +967,7 @@ shared_examples_for 'Msf::DBManager::ImportMsfXml' do
 			end
 
 			let(:source) do
-				xml.web_page do
+				xml.web_vuln do
 					xml.category(
 							web_vuln_attributes.fetch(:category)
 					)