[NFC] Fix typo in address space conversion example (#1322)

Author: wenju-he

OpenCL_C.txt

@@ -3336,7 +3336,7 @@ constant int *cp;
 
 int *p;
 gp = (global int *)lp;  // illegal to cast between named address spaces
-p = (int *)lp;          // legal to cast from global to generic
+p = (int *)gp;          // legal to cast from global to generic
 gp = (global int*)p;    // legal to cast from generic to global
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