Apply suggestions from code review

Co-authored-by: S. B. Tam <[email protected]>
Co-authored-by: Johel Ernesto Guerrero Peña <[email protected]>

Author: 3 people

source/preprocessor.tex

@@ -520,14 +520,14 @@
 \terminal{\# ifndef  } identifier new-line \opt{group}\br
 \indextext{\idxcode{\#ifndef}}%
 \terminal{\# elifdef } identifier new-line \opt{group}\br
-\indextext{\idxcode{\#ifdef}}%
+\indextext{\idxcode{\#elifdef}}%
 \terminal{\# elifndef} identifier new-line \opt{group}
-\indextext{\idxcode{\#ifndef}}%
+\indextext{\idxcode{\#elifndef}}%
 \end{ncsimplebnf}
 check whether the identifier is or is not currently defined as a macro name.
 Their conditions are equivalent to
 \tcode{\#if} \tcode{defined} \grammarterm{identifier},
-\tcode{\#if} \tcode{!defined} \grammarterm{identifier}
+\tcode{\#if} \tcode{!defined} \grammarterm{identifier},
 \tcode{\#elif} \tcode{defined} \grammarterm{identifier}, and
 \tcode{\#elif} \tcode{!defined} \grammarterm{identifier},
 respectively.