Doc: fix description of how to use src/tutorial files.

 Doc: fix description of how to use src/tutorial files.
 
 The separate "cd" command before invoking psql made sense (or at least
 I thought so) when it was added in commit ed1939332.  But 4e3a61635
 removed the supporting text that explained when to use it, making it
 just confusing.  So drop it.
 
 Also switch from four-dot to three-dot filler for the unsupplied
 part of the path, since at least one person has read the four-dot
 filler as a typo for "../..".  And fix these/those inconsistency.
 
 Discussion: https://postgr.es/m/160837647714.673.5195186835607800484@wrigleys.postgresql.org