grammar fix

Author: avehtari

src/reference-manual/statements.qmd

@@ -14,7 +14,7 @@ are used --- if they do not, the behavior is undefined.
 
 The basis of Stan's execution is the evaluation of a log probability
 function (specifically, a log probability density function) for a given
-set of (real-valued) parameters. Log probability function can be
+set of (real-valued) parameters. Log probability functions can be
 constructed by using distribution statements and log probability increment 
 statements.  Statements may be grouped
 into sequences and into for-each loops.  In addition, Stan allows