revert changes to libjpeg paths back to /usr/

based on PR feedback about this document being aimed at Linux

Author: jpsim

Documentation/SystemModules.md

@@ -24,11 +24,13 @@ To `import CJPEG` the package manager requires
 that the JPEG library has been installed by a system packager (eg. `apt`, `brew`, `yum`, etc.).
 The following files from the JPEG system-package are of interest:
 
-    /usr/local/lib/libjpeg.so      # .dylib on OS X
-    /usr/local/include/jpeglib.h
+    /usr/lib/libjpeg.so      # .dylib on OS X
+    /usr/include/jpeglib.h
 
 Swift packages that provide module maps for system libraries are handled differently from regular Swift packages.
 
+Note that `brew` will install this package in `/usr/local/` rather than `/usr/`.
+
 Create a directory called `CJPEG` parallel to the `example` directory and create a file called `module.modulemap`:
 
     example$ cd ..
@@ -39,7 +41,7 @@ Create a directory called `CJPEG` parallel to the `example` directory and create
 Edit the `module.modulemap` so it consists of the following:
 
     module CJPEG [system] {
-        header "/usr/local/include/jpeglib.h"
+        header "/usr/include/jpeglib.h"
         link "jpeg"
         export *
     }
@@ -180,9 +182,9 @@ And run it:
 Please note that on some platforms, the above steps fail with:
 
     <module-includes>:1:10: note: in file included from <module-includes>:1:
-    #include "/usr/local/include/jpeglib.h"
+    #include "/usr/include/jpeglib.h"
              ^
-    /usr/local/include/jpeglib.h:792:3: error: unknown type name 'size_t'
+    /usr/include/jpeglib.h:792:3: error: unknown type name 'size_t'
       size_t free_in_buffer;        /* # of byte spaces remaining in buffer */
       ^