CMake: fix name of soundtouch pkgconfig file

Files installed by the soundtouch-devel package in Fedora 34:
❯ rpm -ql soundtouch-devel
/usr/include/soundtouch
/usr/include/soundtouch/BPMDetect.h
/usr/include/soundtouch/FIFOSampleBuffer.h
/usr/include/soundtouch/FIFOSamplePipe.h
/usr/include/soundtouch/STTypes.h
/usr/include/soundtouch/SoundTouch.h
/usr/include/soundtouch/soundtouch_config.h
/usr/lib64/libSoundTouch.so
/usr/lib64/pkgconfig/libSoundTouch.pc
/usr/lib64/pkgconfig/soundtouch-1.0.pc
/usr/lib64/pkgconfig/soundtouch.pc
/usr/share/aclocal/soundtouch.m4

cmake/modules/FindSoundTouch.cmake

@@ -45,7 +45,7 @@ The following cache variables may also be set:
 
 find_package(PkgConfig QUIET)
 if(PkgConfig_FOUND)
-  pkg_check_modules(PC_SoundTouch QUIET SoundTouch>=2.1.1)
+  pkg_check_modules(PC_SoundTouch QUIET soundtouch>=2.1.1)
 endif()
 
 find_path(SoundTouch_INCLUDE_DIR