[stmt.if] Add missing comma after conditional clause (#7061)

Author: Eisenwave

source/statements.tex

@@ -267,7 +267,7 @@
 \indextext{statement!\idxcode{if}}
 
 \pnum
-If the condition\iref{stmt.pre} yields \tcode{true} the first
+If the condition\iref{stmt.pre} yields \tcode{true}, the first
 substatement is executed. If the \keyword{else} part of the selection
 statement is present and the condition yields \tcode{false}, the second
 substatement is executed. If the first substatement is reached via a