Correct typo and URL (#491)

Distributed Weakness filing went dark last year
Instead use, the official pages at mitre corp.

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

CONTRIBUTING.md

@@ -18,7 +18,8 @@ this repository containing the new advisory:
 Feel free to do either or both of these as you see fit (we recommend you do both):
 
 4. [Yank] the affected versions of the crate.
-5. Request a CVE for your vulnerability: https://iwantacve.org/
+5. Request a CVE for your vulnerability. See for details:
+   https://cve.mitre.org/cve/request_id.html and https://cveform.mitre.org .
    Alternatively, you can create a GitHub Security Advisory (GHSA) and let them request
    a CVE for you. In this case, you can add the GHSA ID to the RustSec advisory via the
    `aliases` field.

README.md

@@ -29,7 +29,7 @@ See [CONTRIBUTING.md] for more information.
 See [EXAMPLE_ADVISORY.md] for a template.
 
 Advisories are formatted in [Markdown] with [TOML] "front matter".
-Below is the scehma of the "front matter" section of an advisory:
+Below is the schema of the "front matter" section of an advisory:
 
 ```toml
 # Before you submit a PR using this template, **please delete the comments**