Fix typo in comment. Was copied directly from:

https://github.com/jtojnar/cmake-snips/blob/master/CMakeScripts/JoinPaths.cmake
Revert change to exec_prefix. Ends up as same value.

cmake/JoinPaths.cmake

@@ -1,5 +1,5 @@
 # This module provides function for joining paths
-# known from from most languages
+# known from most languages
 #
 # Original license:
 # SPDX-License-Identifier: (MIT OR CC0-1.0)

raylib.pc.in

@@ -1,5 +1,5 @@
 prefix=@CMAKE_INSTALL_PREFIX@
-exec_prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=${prefix}
 libdir=@libdir_for_pc_file@
 includedir=@includedir_for_pc_file@