docs: reference-guide: Fix link to count function

This fixes the link to the count() function under the Frequency
Counting section.
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>

docs/reference_guide.md

@@ -951,7 +951,7 @@ slept for 1000 ms
 
 ## 4. `count()`: Frequency Counting
 
-This is provided by the count() function: see the [Count](#2-count) section.
+This is provided by the count() function: see the [Count](#2-count-count) section.
 
 ## 5. `hist()`, `lhist()`: Histograms