doc: add --update-assert-snapshot to node.1

Author: cjihrig

doc/node.1

@@ -486,6 +486,9 @@ Track heap object allocations for heap snapshots.
 .It Fl -unhandled-rejections=mode
 Define the behavior for unhandled rejections. Can be one of `strict` (raise an error), `warn` (enforce warnings) or `none` (silence warnings).
 .
+.It Fl -update-assert-snapshot
+Updates snapshot files used by `assert.snapshot()`.
+.
 .It Fl -use-bundled-ca , Fl -use-openssl-ca
 Use bundled Mozilla CA store as supplied by current Node.js version or use OpenSSL's default CA store.
 The default store is selectable at build-time.