Fix incorrect file reference in guc.h

GucSource_Names was documented as being in guc.c, but since 0a20ff54f5
it is located in guc_tables.c.  The reference to the location of
GucSource_Names is important, as GucSource needs to be kept in sync with
GucSource_Names.

Author: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/CAKFQuwYPgAHWPYjPzK7iXzhSZ6MKR8w20_Nz7ZXpOvx=kZbs7A@mail.gmail.com
Backpatch-through: 16

src/include/utils/guc.h

@@ -106,7 +106,7 @@ typedef enum
  * will show as "default" in pg_settings.  If there is a specific reason not
  * to want that, use source == PGC_S_OVERRIDE.
  *
- * NB: see GucSource_Names in guc.c if you change this.
+ * NB: see GucSource_Names in guc_tables.c if you change this.
  */
 typedef enum
 {