fix sensor example (doc)

plus add build instructions (in doc/counter_ex.c)

doc/counter_ex.h

+/*
+ * Example of a basic counter: an accumulator
+**/
+
+unsigned int init_acc(char *, void **);
+unsigned int get_acc(uint64_t *results, void *);
+void clean_acc(void *);
+void label_acc(char **labels, void *);
+
+Sensor rapl = {
+    .init = init_acc,
+    .get = get_acc,
+    .clean = clean_acc,
+    .label = label_acc,
+    .nb_opt = 1,
+};
+
+Optparse rapl_opt[1] = {
+    {
+        .longname = "accumulator",
+        .shortname = 'a',
+        .argtype = OPTPARSE_NONE,		/* OPTPARSE_NONE / OPTPARSE_OPTIONAL / OPTPARSE_REQUIRED */
+        .usage_arg = NULL,
+        .usage_msg = "dumb accumulator",
+    },
+};