Fix grammatical typos around possessive "its"

Some places spelled it "it's", which is short for "it is".
In passing, fix a couple other nearby grammatical errors.

Author: Jacob Brazeal <jacob.brazeal@gmail.com>
Discussion: https://postgr.es/m/CA+COZaAO8g1KJCV0T48=CkJMjAnnfTGLWOATz+2aCh40c2Nm+g@mail.gmail.com

src/backend/utils/adt/formatting.c

@@ -5561,7 +5561,7 @@ NUM_numpart_from_char(NUMProc *Np, int id, int input_len)
 		}
 
 		/*
-		 * try read non-locale sign, it's happen only if format is not exact
+		 * try read non-locale sign, which happens only if format is not exact
 		 * and we cannot determine sign position of MI/PL/SG, an example:
 		 *
 		 * FM9.999999MI			   -> 5.01-

src/backend/utils/adt/jsonpath_exec.c

@@ -77,7 +77,7 @@
 #include "utils/timestamp.h"
 
 /*
- * Represents "base object" and it's "id" for .keyvalue() evaluation.
+ * Represents "base object" and its "id" for .keyvalue() evaluation.
  */
 typedef struct JsonBaseObjectInfo
 {

src/backend/utils/adt/tsrank.c

@@ -812,7 +812,7 @@ get_docrep(TSVector txt, QueryRepresentation *qr, int *doclen)
 		qsort(doc, cur, sizeof(DocRepresentation), compareDocR);
 
 		/*
-		 * Join QueryItem per WordEntry and it's position
+		 * Join QueryItem per WordEntry and its position
 		 */
 		storage.pos = doc->pos;
 		storage.data.query.items = palloc(sizeof(QueryItem *) * qr->query->size);