doc: Fixes for signal, image, audio, and OOP chapters (bug #52844). * oop.txi: Remove duplicate word "can". * package.txi: Change @example code to find current_installation_directory so that it would actually run. Rewrite BuildRequires item. * plot.txi: Re-phrase language about when Contour group objects are created. * fft.cc (Ffft, Fifft): Use the same variable name 'x' in docstring as in @deftypefn. Add period to final sentence of docstring. * fft2.cc (Ffft2): Add period to final sentence of docstring. * lin2mu.m: Add output variable 'y' to @deftypefn. * mu2lin.m: Add output variable 'y' to @deftypefn. Space out mathematical expression for better readibility. * ind2gray.m: Use the same variable name 'map' in docstring as in @deftypefn. * rgb2ind.m: Add period to final sentence of docstring. * periodogram.m: Use ':' operator rather than '..' to indicate range.

--- a/doc/interpreter/oop.txi	Mon Jan 08 12:24:33 2018 -0800
+++ b/doc/interpreter/oop.txi	Tue Jan 09 21:10:38 2018 -0800
@@ -429,7 +429,7 @@
 
 Functions that are of particular interest for overloading are the class
 conversion functions such as @code{double}.  Overloading these functions allows
-the @code{cast} function to work with a user class.  It can also can aid in the
+the @code{cast} function to work with a user class.  It can also aid in the
 use of a class object with methods and functions from other classes since the
 object can be transformed to the requisite input form for the new function.
 An example @code{double} function for the polynomial class might look like