Remove the libyara from path libyara/Makefile.am

The file is no longer present on that path and moved to root of project.
Fixing the docs so it does'nt cause confusion to others like it did to
me.

docs/writingmodules.rst

@@ -202,7 +202,7 @@ In our case the resulting *module_list* is::
 
 The second step is modifying the *Makefile.am* to tell the *make* program that
 the source code for your module must be compiled and linked into YARA. At the
-very beginning of *libyara/Makefile.am* you'll find this::
+very beginning of *Makefile.am* you'll find this::
 
     MODULES =  libyara/modules/tests/tests.c
     MODULES += libyara/modules/pe/pe.c