docs: address review feedback on counter example and table header

Revert quick-start counter example to use 'my_failures' (no _total suffix)
since the library appends it automatically. Rename 'Value goes' column to
'Behavior' in the metric type overview table for clarity across all six types.

Signed-off-by: k1chik <107162115+k1chik@users.noreply.github.com>

Author: k1chik

docs/content/instrumenting/_index.md

@@ -5,7 +5,7 @@ weight: 2
 
 Six metric types are available. Pick based on what your value does:
 
-| Type | Value goes | Use for |
+| Type | Behavior | Use for |
 |------|-----------|---------|
 | [Counter](counter/) | only up | requests served, errors, bytes sent |
 | [Gauge](gauge/) | up and down | queue depth, active connections, memory usage |

docs/content/instrumenting/counter.md

@@ -10,7 +10,7 @@ If your value can go down, use a [Gauge](../gauge/) instead.
 
 ```python
 from prometheus_client import Counter
-c = Counter('my_failures_total', 'Description of counter')
+c = Counter('my_failures', 'Description of counter')
 c.inc()     # Increment by 1
 c.inc(1.6)  # Increment by given value
 ```