Fix collection of typos in the code and the documentation

Some words were duplicated while other places were grammatically
incorrect, including one variable name in the code.

Author: Otto Kekalainen, Justin Pryzby
Discussion: https://postgr.es/m/7DDBEFC5-09B6-4325-B942-B563D1A24BDC@amazon.com

src/backend/access/heap/pruneheap.c

@@ -69,7 +69,7 @@ typedef struct
 	/*
 	 * Tuple visibility is only computed once for each tuple, for correctness
 	 * and efficiency reasons; see comment in heap_page_prune() for
-	 * details. This is of type int8[,] intead of HTSV_Result[], so we can use
+	 * details. This is of type int8[,] instead of HTSV_Result[], so we can use
 	 * -1 to indicate no visibility has been computed, e.g. for LP_DEAD items.
 	 *
 	 * Same indexing as ->marked.

src/backend/access/transam/xlog.c

@@ -5870,7 +5870,7 @@ GetRedoRecPtr(void)
  * full-page image to be included in the WAL record.
  *
  * The returned values are cached copies from backend-private memory, and
- * possibly out-of-date or, indeed, uninitalized, in which case they will
+ * possibly out-of-date or, indeed, uninitialized, in which case they will
  * be InvalidXLogRecPtr and false, respectively.  XLogInsertRecord will
  * re-check them against up-to-date values, while holding the WAL insert lock.
  */