'r' history parameter documentation fix

[Jesse Weinstein <jesse@xxxxxxxx>]

In the Modifier's section of the docs, defining the 'r' history
modifier, "Remove a filename extension of the form `.XXX', leaving the
root name." appears to be subtly wrong.

At least in 4.3.10.Debian, (foo=blah.flac; echo ${foo:r}) gives blah, 
although it should't, according to the manual, as .flac isn't of the
form .XXX .  

I think the doc should be changed to "Remove a filename extension
leaving the root name."

Jesse Weinstein