Use name 'withdrawn' instead of 'modified' for Vulnerability struct (#…

…1548)

Followup from #1531, appears to be
the result of a copy-paste error? cc @andrewpollock

Found by seeing [failures when importing
vulnerabilities](https://github.com/pypa/advisory-database/actions/runs/5813635878/job/15761635411)
in pypa/advisory-database

Co-authored-by: Andrew Pollock <andrewpollock@users.noreply.github.com>

vulnfeeds/vulns/vulns.go

@@ -191,7 +191,7 @@ type Reference struct {
 
 type Vulnerability struct {
 	ID         string      `json:"id" yaml:"id"`
-	Withdrawn  string      `json:"withdrawn,omitempty" yaml:"modified,omitempty"`
+	Withdrawn  string      `json:"withdrawn,omitempty" yaml:"withdrawn,omitempty"`
 	Summary    string      `json:"summary,omitempty" yaml:"summary,omitempty"`
 	Severity   []Severity  `json:"severity,omitempty" yaml:"severity,omitempty"`
 	Details    string      `json:"details" yaml:"details"`