Fix typo for canonversion rule (#631)

* Fix typo for canonversion rule

* Update design/mvp/Explainer.md

Co-authored-by: Luke Wagner <mail@lukewagner.name>

Author: chenyan2002

design/mvp/Explainer.md

@@ -2557,6 +2557,7 @@ interfaceversion  ::= '@' <valid semver>
 canonversion      ::= [1-9] [0-9]* 🔗
                     | '0.' [1-9] [0-9]* 🔗
                     | '0.0.' [1-9] [0-9]* 🔗
+                    | '0.0.0' 🔗
 semversuffix      ::= [0-9A-Za-z.+-]* 🔗
 depname           ::= 'unlocked-dep=<' <pkgnamequery> '>'
                     | 'locked-dep=<' <pkgname> '>' ( ',' <hashname> )?