Update 03-environment-variables.pod6

Change FormatCode F to C, since F is not usefully implemented. C<> changes the markup

Author: finanalyst

doc/Programs/03-environment-variables.pod6

@@ -68,7 +68,7 @@ want to avoid the recommendation message upon REPL startup.
 =item C<RAKUDO_DISABLE_MULTILINE> Set to 1, will disable multiline input for the REPL
 
 =item C<RAKUDO_HIST> This specifies the location of the history file used by the
-line editor; the default is F<~/.perl6/rakudo-history>.
+line editor; the default is C<~/.perl6/rakudo-history>.
 
 
 =head2 Other