doc/fftools-common-opts: Fixed a typo in the common arguments list

- Fixed a typo for the -sources argument

Signed-off-by: Mickael Maison <mickael.maison@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/fftools-common-opts.texi

@@ -153,7 +153,7 @@ Show channel names and standard channel layouts.
 Show recognized color names.
 
 @item -sources @var{device}[,@var{opt1}=@var{val1}[,@var{opt2}=@var{val2}]...]
-Show autodetected sources of the intput device.
+Show autodetected sources of the input device.
 Some devices may provide system-dependent source names that cannot be autodetected.
 The returned list cannot be assumed to be always complete.
 @example