Doc: fix data types of FuncCallContext's call_cntr and max_calls fields.

Commit 23a27b03 widened these from uint32 to uint64, but I overlooked
that the documentation explicitly showed them as uint32.  Per report
from Vicky Vergara.

Report: <20161111135422.8761.36733@wrigleys.postgresql.org>

doc/src/sgml/xfunc.sgml

@@ -2870,7 +2870,7 @@ HeapTupleGetDatum(HeapTuple tuple)
      is used to hold a pointer to <structname>FuncCallContext</>
      across calls.
 <programlisting>
-typedef struct
+typedef struct FuncCallContext
 {
     /*
      * Number of times we've been called before
@@ -2878,7 +2878,7 @@ typedef struct
      * call_cntr is initialized to 0 for you by SRF_FIRSTCALL_INIT(), and
      * incremented for you every time SRF_RETURN_NEXT() is called.
      */
-    uint32 call_cntr;
+    uint64 call_cntr;
 
     /*
      * OPTIONAL maximum number of calls
@@ -2887,7 +2887,7 @@ typedef struct
      * If not set, you must provide alternative means to know when the
      * function is done.
      */
-    uint32 max_calls;
+    uint64 max_calls;
 
     /*
      * OPTIONAL pointer to result slot