[stmt.while] Add comma after introductory phrase

Author: Eisenwave

source/statements.tex

@@ -550,7 +550,7 @@
 \indextext{statement!\idxcode{while}}
 
 \pnum
-In the \keyword{while} statement the substatement is executed repeatedly
+In the \keyword{while} statement, the substatement is executed repeatedly
 until the value of the condition\iref{stmt.pre} becomes
 \tcode{false}. The test takes place before each execution of the
 substatement.