Update reference to sampling algorithm in analyze.c

Commit 83e176ec moved row sampling functions from analyze.c to
utils/misc/sampling.c, but failed to update comment referring to
the sampling algorithm from Jeff Vitter's paper. Correct the
comment by pointing to utils/misc/sampling.c.

Author: Etsuro Fujita
Discussion: https://postgr.es/m/CAPmGK154gp%2BQd%3DcorQOv%2BPmbyVyZBjp_%2Bhb766UJeD1e_ie6XQ%40mail.gmail.com

src/backend/commands/analyze.c

@@ -1048,13 +1048,13 @@ acquire_sample_rows(Relation onerel, int elevel,
 			 * The first targrows sample rows are simply copied into the
 			 * reservoir. Then we start replacing tuples in the sample until
 			 * we reach the end of the relation.  This algorithm is from Jeff
-			 * Vitter's paper (see full citation below). It works by
-			 * repeatedly computing the number of tuples to skip before
-			 * selecting a tuple, which replaces a randomly chosen element of
-			 * the reservoir (current set of tuples).  At all times the
-			 * reservoir is a true random sample of the tuples we've passed
-			 * over so far, so when we fall off the end of the relation we're
-			 * done.
+			 * Vitter's paper (see full citation in utils/misc/sampling.c). It
+			 * works by repeatedly computing the number of tuples to skip
+			 * before selecting a tuple, which replaces a randomly chosen
+			 * element of the reservoir (current set of tuples).  At all times
+			 * the reservoir is a true random sample of the tuples we've
+			 * passed over so far, so when we fall off the end of the relation
+			 * we're done.
 			 */
 			if (numrows < targrows)
 				rows[numrows++] = ExecCopySlotHeapTuple(slot);