doc/filters: fix typo

Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -1202,7 +1202,7 @@ Each key points list need to be defined using the following syntax:
 @code{x0/y0 x1/y1 x2/y2 ...}.
 
 The input values must be in strictly increasing order but the transfer
-function does not have to me monotonically rising.
+function does not have to be monotonically rising.
 The point @code{0/0} is assumed but may be overridden (by @code{0/out-dBn}).
 Typical values for the transfer function are @code{-70/-70 -60/-20}.