Remove obsolete mention of backslashes as escapes

The documentation in
https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-DOLLAR-QUOTING
says that

 "While the standard syntax for specifying string constants is usually convenient,
  it can be difficult to understand when the desired string contains many single
  quotes or backslashes, since each of those must be doubled."

But this has been obsolete ever since version 9.1, when "standard_conforming_strings"
started to default to "on".  It has confused at least one reader:
https://dba.stackexchange.com/q/325850/176905

So I propose to remove the mention of backslashes there.

Yours,
Laurenz Albe