[namespace.qual] Add missing \tcode formatting to X (#3975)

Author: sdkrystian

source/basic.tex

@@ -2216,7 +2216,7 @@
 
 \pnum
 Given \tcode{X::m} (where \tcode{X} is a user-declared namespace), or
-given \tcode{::m} (where X is the global namespace), if
+given \tcode{::m} (where \tcode{X} is the global namespace), if
 $S(X, m)$ is the empty set, the program is ill-formed. Otherwise, if
 $S(X, m)$ has exactly one member, or if the context of the reference is
 a \grammarterm{using-declaration}\iref{namespace.udecl}, $S(X, m)$