added 'boolean' to the grammars for the *SLs, fixing an emailed bug report from Klause Ostermann. -- To avoid the 'true, '(true) etc issue I ommitted boolean from the [quasi]quoted grammar, which makes the current behavior kind of correct.

collects/scribblings/htdp-langs/advanced.scrbl

@@ -53,6 +53,7 @@
       (code:line @#,elem{@racketvalfont{'}@racket[_quoted]})
       (code:line @#,elem{@racketvalfont{`}@racket[_quasiquoted]})
       number
+      boolean
       string
       character]
 [choice (code:line name)

collects/scribblings/htdp-langs/beginner-abbr.scrbl

@@ -32,6 +32,7 @@
       (code:line @#,elem{@racketvalfont{'}@racket[_quoted]})
       (code:line @#,elem{@racketvalfont{`}@racket[_quasiquoted]})
       number
+      boolean	
       string
       character]
 ]

collects/scribblings/htdp-langs/beginner.scrbl

@@ -29,6 +29,7 @@
       name
       (code:line @#,elem{@racketvalfont{'}@racket[name]})
       number
+      boolean 
       string
       character]
 ]

collects/scribblings/htdp-langs/intermediate-lambda.scrbl

@@ -36,6 +36,7 @@
       (code:line @#,elem{@racketvalfont{'}@racket[_quoted]})
       (code:line @#,elem{@racketvalfont{`}@racket[_quasiquoted]})
       number
+      boolean 
       string
       character]
 ]

collects/scribblings/htdp-langs/intermediate.scrbl

@@ -35,6 +35,7 @@
       (code:line @#,elem{@racketvalfont{'}@racket[_quoted]})
       (code:line @#,elem{@racketvalfont{`}@racket[_quasiquoted]})
       number
+      boolean 
       string
       character]
 [expr-for-let (lambda (variable variable ...) expr)

collects/scribblings/htdp-langs/std-grammar.rkt

@@ -74,6 +74,9 @@ not including a space or one of the following:}
 @t{A @racket[_number] is a number such as @racket[123], @racket[3/2], or
 @racket[5.5].}
 
+@t{A @racket[_boolean] is one of: @racket[true], @racket[false],
+@racket[#t], @racket[#f], @code{#true}, or @code{#false}.}
+
 @t{A @racket[_symbol] is a quote character followed by a name. A
 symbol is a value, just like 0 or empty.}