Fix "MUST not" to "MUST NOT".

From the idnids report:

  == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD',
     or 'RECOMMENDED' is not an accepted usage according to RFC 2119.  Please
     use uppercase 'NOT' together with RFC 2119 keywords (if that is what you
     mean).

     Found 'MUST not' in this paragraph:

     *  Values from 0 to 65000 are allocated following a First-Come
     First-Served policy (Section 4.4 of [RFC8126]).  Values in the ranges
     0-10, 50-51, and 98-301 are already assigned; values in the ranges 11-49
     and 52-97 MUST not be assigned.

Author: guyharris

draft-ietf-opsawg-pcaplinktype.md

@@ -697,7 +697,7 @@ The policy allocation for the LinkType values is as follows:
 * Values from 0 to 65000 are allocated following a First-Come
   First-Served policy (Section 4.4 of [RFC8126]). Values in the ranges
   0-10, 50-51, and 98-301 are already assigned; values in the ranges
-  11-49 and 52-97 MUST not be assigned.
+  11-49 and 52-97 MUST NOT be assigned.
 
 * Values from 65001 to 65535 are reserved for Experimental Use ({{Section 4.2 of !RFC8126}}).