Fix typo with RFC reference number

4548 should have been 4648, as per https://github.com/json-schema-org/json-schema-spec/pull/604/files

Author: Relequestual

jsonschema-validation.xml

@@ -765,7 +765,7 @@
                 <t>
                     Possible values for this property are listed in
                     <xref target="RFC2045">RFC 2045, Sec 6.1</xref> and
-                    <xref target="RFC4648">RFC 4548</xref>.  For "base64", which is defined
+                    <xref target="RFC4648">RFC 4648</xref>.  For "base64", which is defined
                     in both RFCs, the definition in RFC 4648, which removes line length
                     limitations, SHOULD be used, as various other specifications have
                     mandated different lengths.  Note that line lengths within a string