[cpp.concat] In example code comment, remove unnecessary line break and format code as code. (#1377)

Author: Eelis

source/preprocessor.tex

@@ -866,8 +866,7 @@
 #define mkstr(a) # a
 #define in_between(a) mkstr(a)
 #define join(c, d) in_between(c hash_hash d)
-char p[] = join(x, y);          // equivalent to
-                                // char p[] = "x \#\# y";
+char p[] = join(x, y);          // equivalent to \tcode{char p[] = "x \#\# y";}
 \end{codeblock}
 
 The expansion produces, at various stages: